You

Choose your loadout carefully, the Citizen Defense tower is best manned by an Operator or a Hunter. Make sure you select the Ammo Station utility perk, and if you are an Operator the C-32 Chi-Hong Grenade Launcher as your primary weapon. If you are playing a different class (or lack the needed levels) then you will need to find a Rocket Launcher during the missions. These are usually rewarded after completing the "Repair the generator" and "Defend the beacon" bonus missions; remember: real citizen swaps to their pistol before collecting a Rocket Launcher, that way you keep your primary weapon.

If you want an "ideal" loadout, we recommend this:

Grenade Launcher and/or Rocket Launcher

You can equip a grenade launcher as an Operator. Finding a Rocket Launcher comes down to luck. Having neither does not immediately invalidate the tower, but makes it much weaker since you are now vulnerable to Gunner bugs and don't do as much AoE damage. So work hard to grab a Rocket Launcher as soon as you can.

Ammo Station (Fabricator)

The ammunition fabricator (aka Ammo Station), is a deployable utility structure that allows anyone to reload all their primary and secondary weapons fully up to 5 times. The best way to use the ammo station is to completely unload all your grenades or rockets into bugs and then collect a full resupply. You can deploy up to two ammo stations, but keep one in reserve just in case it gets destroyed by a wayward explosive. Having your own personal ammo supply in a tower is what makes you a true blue citizen of the federation!

Tower

The tower is a very underrated base building since it allows for near complete immunity to bug melee attacks when placed behind a wall or built inside a bunker. To build a tower you need equip your Build Tool (B), switch it to "place" mode (R), select the tower by navigating the UI with (Q) / (E), with the blueprint selected make sure the ladder is facing inwards (mousewheel scroll to rotate), left click the blueprint to confirm it, switch your build tool back to repair mode (R), and then hold down LMB to build the tower.