I didn't see anyone else make a post on this so i thought i might.
Currently in game,there is only one dedicated defensive structure,that being a measle 12.7mm Pillbox.This does offer some variation in gameplay as it often requires a good degree of munitions,or dedicated bunker busting munitions to take them down.This structure is mainly capable of engaging ground targets,given its limited elevation it doesn't pose much of a risk for aircraft.Even if for most occasions,fixed emplacements are always inferior to mobile ones,i wanted to highlight a few different possibilities that future structures could have if the devs entretained the idea of adding them in.Their roles in battle would vary slightly,so i've decided to divide them into three categories:
Anti-Ground:
This takes the shape of pillboxes,bunkers,artillery emplacements and even ground based cruise missile sites.Their aim would be to deter,deny and/or slow down the advancements of enemies,with perhaps some offensive capabilities too,for the cruise missiles.
Anti-Air:
These mainly range from Radar sites,Launch sites for SAM munitions,Anti Air Artillery,and CIWS.These could both serve as central pillars to a team's air defense,or small stopgaps that would aid in plugging easily exploitable routes (As in flak sites for low flying aircraft that lob nukes late game,Reinforced Radar sites around which much of the intel of a team would be provided,and so on.)
Anti-Ship:
These easily fall under either Coastal Artillery,or Ground Launched Anti Ship missiles.The former would often only work to cover straits and critical passages were enemy ships,often landing ships or small vessels would try to cross,and the latter more meant as a way for pressure to be exerted against unprotected vessels.This latter one would probably have to wait until there's a bit more variety in terms of naval Units.
As always,let me know if i missed anything that you think is needed too!