Wall

Walls are extremely useful for protecting any base. Build a as many walls as possible. Instead of surrounding your entire base with walls, try to create sections, that way the enemies will have to break through multiple layers of walls in order to destroy your base. Note that air units ignore them and some ground units can shoot over them, but most units have to destroy walls to break their way into the base. Placing building outside walls is disastrous because the attackers can reach it easily. Don't keep your walls at lvl 1. If you do, a good hannibal can destroy it with one shot. Robotic mines are made to destroy walls. If you upgrade your walls enough, it will take several Robotic Mines to take down your walls.