"Turtling" is a tactical choice that emphasizes building a strongly defended base, rather than focusing on offensive power. In the eventuality of a military stalemate, the "turtle" is much more likely to survive an assault on his base than someone who focuses on rushing. The viability of this tactic depends upon certain circumstances, including the power of the base defensive structures of a particular faction.
While a expert turtle would be able to outlast his opponents, this strategy has several drawbacks. The requisite walls and base defenses take time to set up, and a blitz with tanks could likely penetrate an enemy's defenses if it occurred early enough. Also, turtling is fairly expensive, and the late-battle focus on defense can cost a military detachment due to the lack of collected reconnaissance and difficulty replacing depleted resources. Also, the best defenders are not always good at offense, and overly focusing on building an invulnerable base can lead to an atrophied strike capability. In addition, the most powerful base defenses are also the most power-hungry, and if the generators on a base are disabled, a turtle's plans may quickly fall apart. Finally, weapons of mass destruction can punch a hole in a base's defense, no matter fortified they are. Because the turtle has enough time to build up his base, he can make as much of these he wants.
Of the two factions in the Great World War II, Soviet forces were better suited to turtling, owing it to their Tesla coil base defense. Outranging all Allied units except the Cruiser, Soviet Coils in large enough amounts could easily halt any ground advance by their opponents. The only real danger was against Allied Aircraft. Sufficient numbers of Apache Gunships could tear down a Tesla coil network, or the power plants required to keep the coils active. During the war, the only decent choice for a passive barrier was the concrete wall. Other Soviet defenses included the Flame tower, which devastating against infantry, but had an extremely short range, and the SAM site, an anti-aircraft missile launcher, that possessed a decent range and energy consumption, but was somewhat lacking in destructive power.
The Allies, unlike their opponents, lacked the proper tools to correctly turtle. Soviet FROG-7s had the range necessary to take out any Allied base defense, from well outside reprisal range, and up close, the turret was only moderately powerful, and unable to stand up to the raw destructive power Soviet tesla coils. However, Allied turrets and infantry slaughtering pillboxes were significantly less power hungry than Tesla Coils, which allowed them to remain operational even at low power conditions. The Allies had significantly more powerful AA than the Soviets in their AA guns (probably because while the only flying Allied threats were Apaches, the Soviets had Yaks, missile-toting MiGs and Hind gunships for the Allies to worry about,) and, like the Soviets, physical barriers took the form of concrete walls.
The Brotherhood of Nod, especially during the earlier wars, was quite capable of turtling, due in large part to the mighty Obelisk of Light, a long range directed energy weapon that outranged any GDI opposition in the First Tiberium War. Nod also possessed gun turrets, similar to those of the Allies, but commanders often preferred the extra power and range of the Obelisk. Against aircraft, Nod used SAM sites.
During the Second Tiberium War, Nod supplemented its Obelisks with smaller laser turrets, which were highly effective against infantry, but lacked the range and power needed to successfully stave off tanks and walkers. Nod continued using SAMs against aircraft.
Even in the Third Tiberium War, Nod's potent hub defense system of anti-aircraft SAMs, infantry annihilating shredder turrets, and laser turrets (the design of which being optimized to destroy vehicles, at the expense of their infantry killing ability) combined with an upgraded arsenal of Obelisks, made turtling a viable option for many Nod bases. Also upgrades include a fourth turret may make Hubs fight back when the three turrets are down.
In terms of barriers, Nod relied on concrete walls, alongside high-tech laser fence systems that posed an impediment to enemy advances.
Reckoners deploy into optional bunkers and are pretty effective and not even grenades can clear 'em.
In the final War the Nod must deploy a Defense Crawler to build base defenses and the Obelisk can now hit air units.
The rogue Nod AI, CABAL, utilized formidable defensive measures at his Core. Improving upon the basic design of the Obelisk, CABAL created the Obelisk of Darkness, which would vaporize any enemy aircraft within range, and the CABAL Obelisk, which possessed improved armor and recharge rate compared to a standard Nod Obelisk. CABAL also used a Firestorm Barrier, that could only be disabled with the capture of six control centers. Artillery were ineffective and large numbers of tanks or walkers is required for the rogue Nod AIs heavily defended fort.
The Forgotten used old towers and old Obelisks as their base defenses.
In the First Tiberium War, GDI suffered as Nod possessed superior turtle-defeating artillery, and it's best base defenses, the Advanced Guard Tower paled in comparison to the Obelisk, though its versatility against infantry, vehicles, and aircraft still made it a force to be reckoned with. Guard Towers are short ranged structures for the infantry and light vehicles.
In the Second Tiberium War, GDI had much better defenses which, if deployed correctly makes turtling a viable option. The Firestorm barrier presented itself as a uniquely powerful defense, impenetrable to nearly any attack, but it was expensive and time consuming to set up, and the component tower was versatile, but its upgrades still lacked the power of the Nod Obelisk. The available upgrades for the tower were the Vulcan Cannon Upgrade, for use against infantry, the RPG Upgrade, which could damage vehicles, and the SAM Upgrade, which was useful against aircraft. Component Towers also had one advantage over its Nod counterparts: it could be integrated into the concrete walls. This proved to be a double edged sword: while it can hit any unit regardless of where the unit was (the Nod Laser could not hit a unit who got behind the wall), if the tower was destroyed, the large blast of the exploding magazines would collapse adjacent walls, not to mention the massive gap left by the tower itself. GDI could also deploy Juggernauts which could act as a powerful, though inaccurate defense and good counter against enemy artillery.
By the Third Tiberium War however, GDI Sonic emitters were capable of quickly dismantling anything on the ground with splash damage, and made turtling viable for GDI. The lower tier of GDI base defense included AA batteries, which were also able to make short work of aircraft, watchtowers to shred infantry, and Guardian cannons to defeat tanks. Rigs can deploy into Battle Bases to increase base defenses and repair tanks (and other Rigs) nearby. The Battle Base effectively functions as a combination of two Guardian Cannons and a Pitbull's missile pack in terms of firepower, but needs no base power to function and can "undeploy" and move as the situation demands (just as Rifle Squads could leave their foxholes, advance and dig in again; the two units work well together, as the dug-in Riflemen defeat the Infantry that the Battle Base doesn't) Upgrades for the Battle Base include AP ammunition, and Railguns is recommended.
At the Cheyenne Mountain Complex, GDI had energy Barriers to protect the Tacitus but they were destroyed by LEGION as raided and stole the Tacitus. LEGION afterwards just retreated with the artifact instead of fighting the massive GDI Army that will later on return to reinforce those defenses.
Riflemen squads can build bunkers to hold off small attacks from engineers or Militants but foxholes can be cleared by Flame Tanks.
In the Final War GDI must deploy a Defense Crawler to build base defenses.
The alien Storm Column is an extremely powerful defense, capable of knocking aircraft out of the sky, and tearing apart many ground units with its artificial Ion storm, making it perhaps the ultimate base defense of the Third Tiberium War. Buzzer Hives housed infantry shredding Buzzers and Plasma Batteries were capable of blasting aircraft out of the sky, and the Photon Cannon, which was useful against tanks. Turtling is definitely a useful measure for the Scrin. Specially because all Scrin units are very expensive.
The Scrin also had Phase Generators to protect their Threshold Towers from assault.