Detector The detectors are very simple to make. They are small and hard to spot, but do not place them in a brightly lit area. Place them in a dark corner or in shadows to prevent them from being destroyed. A detector will sound an alarm at the Marine base when a spider comes close to it. However, it's not the best protection for a base.
Mines are simple to put and they have little cost.
This is the favorite structure of the marines, when a marine runs out of ammo, he can go to the ammo depot. Take great care in where you place it. The ammo depot is dangerous when it's destroyed. The ammo depot can only dispense ammo every 10 seconds, keep that in mind when there's a line of marines trying to use it.