The "You Shall Not pass" strategy is a common method of turtling that is generally not viable in Supreme Commander, excluding a few community maps that are intentionally designed to foster choke defenses. This is distinct from other defensive strategies such as turret creep in that it is more passive and usually performed by inexperienced players.
The "You Shall Not Pass" strategy is a form of turtling which will usually lose the game. It relies on massive amounts of point defense, T2 artillery, and anti-air emplacements, along with defenses such as shields and tactical missile defense, designed to cover any enemy force in an overwhelming amount of firepower. This strategy tends to be popular among newer players, likely because they are less easily able to notice the glaring weaknesses of passive defensive play.
Most obviously, the line is both immobile and an enormous resource drain, meaning that a player's capability in other basic mechanics such as resource acquisition, tech management, and area control (most ironically), will have to be sacrificed in order to funnel resources into the line. When an opponent outflanks the fortifications, they will likely wreck havoc in a player's main base due to a lack of resistance. Furthermore, the line usually puts no pressure on an opponent, meaning that they are able to invest their own resources into any strategy they choose, without worry of attack. If an opponent is able to successfully build a mobile unit, the turtle player is significantly worse off since their entire strategy relied upon a stationary group of buildings and the mobile unit can be anywhere besides the range of the defensive line.
On some maps, such as Thermopylae, which is intentionally built to promote these kinds of strategies (players often disable long range artillery and nukes in such games) a ridiculous line of turrets is nearly the entire meta. The key here is to not to play those games unless that's specifically how you want to play, because Thermopylae is really more about the spectacle of mulching giant robots into an endless stream of lasers.
Positives for 'YOU SHALL NOT PASS LINE"
1. It creates a fallback point on a section of the map, which would require a significant investment for an opponent to a-click past.
2. Point defenses are often more lethal for their cost than mobile attack forces, which means that tanks will suffer significant losses if they don't have artillery support.
3. I an opponent a-clicks into the line, the reclaim from the attack will be in close reach and easily available, which will provide a large mass boost to repair any damage or prepare a counter-attack.
Negatives for "YOU SHALL NOT PASS LINE"
1. In "The Art of War", Sun Tzu says, "A general who attempts to bolster his defenses everywhere will quickly find he has no defenses anywhere". In other words, the "You Shall Not Pass" line cannot be employed everywhere due to its cost, and attempting to do so will only leave weak defenses everywhere. Such a construction project is a huge waste of resources, which could be far better used elsewhere.
2. Such a line is extremely vulnerable to game ending units, specifically heavy artillery and nukes. Without nuke defense, a single nuke placed within the line is likely to be followed by a heavy ground attack, and since a large part of the player's resources were put into the line, he or she is unlikely to be able to effectively defend. Similarly, heavy artillery will slowly chew through shielding until the line is destroyed.
3. A Czar crash will destroy almost any unit in the large blast zone. It is almost impossible to defend against this with stationary defenses, since even if there are enough SAMs to take out the Czar with a single volley, they do not have the range to prevent the wreckage from traveling into the line.
4. A static defense does not put any pressure on the opponent, and so leaves them to eco and come up with a solution to bypass, penetrate, or destroy the line. This effectively gives the opponent control over the game, since the player will endlessly be responding to the opponent's actions.
5. It is difficult to establish sufficient defenses covering a large enough area for the strategy to work, which often means that the player will be vulnerable in the early game, especially if the opponent attempts to go for an early experimental or snipe.
6. The critical disadvantage of this strategy is that it mobile units can accomplish essentially the same thing, but can also be moved to wherever they are needed, and to attack. For this reason alone, as well as those above, it is typically better to rely on mobile units for the main part of your defense, with maybe some stationary defenses for support.
Why Is It More Effective to Avoid Turtling? Edit
Simply put, because anything has more tactical value. When first getting into a game, it's hard to asses the strategic value of each unit. Some units may be built around certain duties on paper, but in practice they need too much support to do as they're intended. Other ideal roles may need a lot of set-up time, and as a general rule, if it takes more than thirty seconds to execute a strategy, that strategy is either a bit of a Hail Mary or it's just never going to work. A good example would be tactical missiles under a stealth field. It takes a while to set up and it relies on your opponent not flying overhead at some point as you're getting the structures built, so although it can be effective, it's not a trick that works with a lot of dependable consistency. Any tactic that relies on your opponent being unaware of your plans for a few minutes can be like that. There's risk and reward.
Turtling provides a very low risk, but sadly, it really achieves no reward. It doesn't allow map control, it doesn't hurt your opponent, it's expensive so it slows down your economy, and ultimately it stops being a question of risk versus reward and more of a question of investment versus returns. A "You Shall Not Pass" defensive line is a huge investment that nets virtually zero returns, excluding, perhaps, a few mass extractors your opponent will have a very hard time reaching.
Mobile units, however, have multiple uses and purposes. By investing in gunships, you can get a variety of positive benefits from them, whether you use them defensively, as air cavalry, for harassment, or even just for scouting and testing defenses. The same goes for most units, although all mobile units are specialized towards their own roles. A defensive turret does one thing: it attacks enemies within its range. The mobile units do numerous things.
Although the mobile units are less expensive and more vulnerable, they have more tactical value and they're more versatile. That means that, although turrets do their singular jobs more effectively than their mobile counterparts, the mobile units still have more value. You're paying less to get more versatility - you're paying less to, in a sense, get a turret that you can simply load into a transport and send straight to the enemy base. The tactical value of mobile units is implicit. It's not something you can describe in terms of mass or energy costs, but having it is essential to winning a game. In blunt terms, static defenses are highly cost-inefficient once you start factoring in strategic considerations.
Using Defenses in a Functional Manner Edit
There is no such thing as a "You Shall Not Pass" defensive line. Your enemy will pass. They'll go around, they'll blow up the line, they'll attack it with experimentals, and so on. It's a huge waste of resources to try to build a line that can't be cracked. So given the weak strategic value of static defenses when used in this manner, how can they best be utilized?
AA Defenses Edit
AA defenses are your most practical defenses and the ones that are built with most frequency by the average player. Although interceptors are more practical when there's time to react with them, the AOE damage provided by T2 Flak cannons can knock chunks out of a bomber squadron. Often times, static AA defenses may not be sufficient to stop a flight pattern from hitting their target, but they will be enough to prevent a second bombing run from the same squadron. Additionally, flak cannons that fire on incoming transports will perform damage to the units being transported, which occasionally results in the death of the payload even if the transport itself doesn't go down.
T3 AA turrets, meanwhile, are capable of intercepting strategic bombers. They're also effective at softening airborne experimentals. However, they are not terribly useful for dealing with large squadrons.
Finally, T1 turrets are functional, but easily the least useful of your three options. They will bring down enemy aircraft, but not with the same reliability as an interceptor, and often times, during the early game, it's more effective to produce mobile AA guns. This rule can actually apply with T2 flak cannons as well, since it's possible to build mobile flak cannons. The only turret that can't be replaced is the T3 AA gun.
Despite that the T1 and T2 turrets are less useful than interceptors and mobile AA, it's a still a good idea to place them around your base for the simple fact that they can bolster your defender's advantage. It's not uncommon to find your enemy has invested more resources into his airforce than you have, and in those cases it could be catastrophic if a superior squadron of interceptors were flown into your base to eliminate your ability to respond to air threats. Having a few turrets around helps prevent this tactic. An inferior force of interceptors that is assisted by AA guns can pull through, forcing your opponent to retreat and recoup losses.
So in these cases, it's observed that the AA guns are actually supporting your air force. Your air force itself is the primary method by which you control the skies, and a small network of AA guns is the safe haven to which they may retreat if the tables turn. As long as you maintain a formidable air force, you should only require a few AA guns placed strategically around your base. The number you'll need is dependent on the size and sprawl of your base, but as long as you have decent coverage, you'll find any attempt to dominate your personal skies being thwarted well enough for your mobile units to deal with the slack.
Point Defenses Edit
Point defenses, like AA guns, should also be supplementary in nature. A T1 point defense is powerful enough to cause real trouble for a T1 army, and it can easily earn back the mass that was spent on it. This fact is common knowledge, and it's why many inexperienced players tend to want to rely on point defenses instead of a mobile ground force. The problem is, the point defense only projects your influence as far as their range, and no further. Even the lowly LABs, meanwhile, can run all across the map, killing engineers and denying mass points.
Consequently, point defenses are most useful for two things: deterring ground harassment and defending hotly contested areas. Though it may seem counter-intuitive, your primary base is not a hotly contested area in most cases. In fact, it's one of the least contested areas of the game because it will spend most of the battle crawling with your own units. If you opponent can build a forward base on top of your own primary base, you have done something terribly, terribly wrong.
Ultimately, you should find yourself building very few point defenses, and if you do so at all, it will often be to assist a forward position that is producing mobile units. Alternatively, you may choose to build point defenses if you see an advancing force you aren't ready for, perhaps spotted during a bomber fly-by or simply by radar. In these cases, the point defenses will be supplementing your army, which is, at that moment, too small to defeat the incoming attackers. The important thing to recognize here is that the point defenses are not going anywhere once they're built, so they absolutely need to be placed only in areas where you know the enemy has to pass through or is going to pass through.
Just as with the AA turrets, point defenses function most usefully as a fall-back position. If the tide of battle turns, your ground forces can retreat to a point defense and benefit from the extra fire power, which will cause a pursuing enemy to fall back. Your mobile ground troops control the ground, and your turrets are a merely safety net in case of catastrophe. If it's some distance between your own base and the enemy, there's little benefit to having a large quantity of point defenses in your primary base, since it's impractical to fall back from the front all the way to your primary base. Think of turrets as rally points, not as a solution to enemy aggression.
Artillery has multiple functions, and very few of them are outright defensive in nature. In most cases, it's actually a superior strategy to build mobile, T3 artillery if you need this kind of force projection. The way you use your artillery also depends on your faction. The Cybran artillery is useful for softening enemy mobile formations, whereas Aeon artillery is better suited for attacking priority targets. One way or the other, they're most commonly built in an attempt to resolve a duel between forward bases.
There are many maps where controlling a particular region can be beneficial, and often, two opponents will build forward bases and attempt to secure these regions. These types of battles can become very bloody and may result in vicious stalemates if neither side can overcome the other's position. The tide may turn back and forth, but each player struggles to overwhelm the other's forward base.
This is when artillery comes into play. Neither side is intent on going anywhere, and the artillery will provide consistent fire which can tip the scales. Defensively, they are a poor option because artillery is very expensive and it's not a good idea to rely on them as a deterrent. They make prime targets for bombing runs, too, because of their high price tag. They can be very powerful on the assist, but they can't be built lightly and it's sometimes difficult to determine if their use is rightly called for over other methods. Often times, they appear after the use of tactical missiles has failed.
Tactical Missiles Edit
Usually the preferred method of static force projection, tactical missiles are purely offensive and should never be built except to threaten static enemy positions. They are both cheaper and have a longer range than artillery, with the one considerable drawback that tactical missile defenses can prevent them from performing their job. As with artillery, they are helpful in resolving duels between forward bases. If the enemy is continually falling back to point defenses that your ground forces can't overcome, a few tactical missiles can clear that problem right up and allow the next wave to finally overwhelm the enemy.
This brings us to the consideration of tactical missile defenses, which should usually be built as early as possible on any forward position that may be threatened by enemy missiles. It typically requires two defenses to halt one missile, so this can quickly become a conflict of escalation. It requires mass and time to build new missiles, but missile defenses can react under any circumstance. These situations therefore often require intel since the two opposing sides will be attempting to exceed each other with regards to these measures.
If tactical missiles can be built covertly, before the enemy can build defenses, the missiles can eliminate numerous high priority targets within their operational range.
One drawback specific to tactical missiles are, that they require a player to launch the missiles remember this. Also by default tactical missile launchers will auto create missiles (this does still require resources).
Torpedo Launchers Edit
Although the same rule generally applies as with other static defenses, torpedo launchers provide fewer benefits once an opponent arrives at T2, where various ships obtain an attack range that exceeds the torpedo launchers. The launchers can be helpful to deter submarines and they may force your opponent to slow their attack, but as far as naval confrontations are concerned, nothing is more effective than a competent navy.
There is one notable exception in the form of the Cybran HARMS. Due to its unique status as a submerged weapon, its high DPS, and its reasonable reach, it can serve as a much more productive fallback position. Its respectable range of 80 means that any naval unit that can attack it will also be within range of the HARMS, except in water shallow enough that damage radius from surface artillery can reach the HARMS.