Turrets play a large role in Strike Tactics gameplay. Because all units are ranged, and because air units are bread and butter, as opposed to support units, wall offs (via buildings) and walls really aren't viable as defensive structures. Instead, turrets are the main defensive structures. Currently there are 3 types of turrets:

Laser turrets - broad role, moderately good against air and ground

Bombard cannons - excels against ground, bad vs. air

Flak cannons - excels against air, cannot target ground

Why can the anti-ground turret shoot at air, while the anti-air cannot shoot at ground units? You'll find a lot of these little discriminations in Strike Tactics. In order for ground units to be viable, they must have enough inherent advantages to neutralize the extreme disadvantages they have in the mobility department against air. Other inherent ground advantages include more firepower, more health and guns which can fire while the unit moves. 

Another turret unit is the Disruptor cannon, which is a large, artillery cannon. Because its incredible range and firepower makes it practical to use offensively, it is under the classification of a super weapon than a defensive structure. 

I am considering implementing wall structures you can build. Except to make walls viable in a game where air units are prevalent, and all units are ranged, the wall structure would perform the following functions:

• Goes into the ground if a friendly unit needs to pass through (see supply depot in starcraft 2)
• Deflect enemy projectiles with shields that raise as an enemy projectile passes through, while allowing friendly projectiles to pass through. 

If implemented correctly, these shield walls could act as a counter to mass-ground forces (movement obstruction) and heavy artillery (such as the Disruptor Cannon). They even could be effective against air units if turrets were strategically placed inside the perimeter of the walls. Air units swooping by would need to be positioned on the right side of the wall to effectively do damage. I am very much committed to finding a place for walls in a futuristic RTS (i.e. an RTS with mostly range/air units), but this I see as a secondary feature which will come later in development, after base unit roles have been established.