Command & Conquer: Red Alert 3/Turtling

Turtling is a tactic that involves heavy base defenses to stall enemy attacks. However, this consumes a lot of money and power, and turtling players are vunerable to attacks from artillery.

Allies
Spectrum Towers are extremely powerful against vehicles when in groups. However, due to the fact that multiple Spectrum Towers can only target 1-2 units at a time, swarms of units can rush in and destroy them.

Multigunner Turrets — The Multigunner Turret is the most versatile defense in the game. Multigunner Turrets garrisoned by Peacekeepers are highly effective against infantry rushes. Able to hit air and ground targets these turrets especially in groups are the perfect way of defending your base from all except artillery or heavy tank attacks.

Soviets
Soviet base defenses are excellent when mixed together:

Sentry Guns are good against infantry and reasonably powerful against light vehicles.

Flak Cannons in groups with overlapping ranges can ground entire air squadrons.

The infamous Tesla Coils can reduce vehicles to molten scrap metal, especially when charged up by Tesla Troopers or Stingrays. Doing this gives Tesla Coils the ability to remain active under low power conditions and provides an increase in range and power.

The Combat Engineer may even create a Bunker at the cost of $500 to help repel most attacks and the Bunker is the only garrisonable structure that can be repaired with the repair mode feature. Garrison the bunker with 3 Flak Troopers and 2 Conscripts.

Empire of the Rising Sun
The Imperial Defender VX can pull double duty with the ability to transition from twin rapid-firing autocannons to rocket pods capable of sending aircraft to their graves.

The Wave-Force Tower is good againist vehicles, although it has a slow rate of fire. The Empire has added flexibility with their defenses because their Nanocores can deploy almost anywhere.