Tower
Once Strong Force tech is known, you can
build Towers instead of Force Fortresses. It gives 150% defense bonus to
Earthly, Native and Amphibious units. When unit in Tower tile loses fight
against attacker, other units in the tile remain alive.
Force Fortress
You can build Force Fortresses once
Force Channeling tech is known.
It gives 100% defense bonus to Earthly, Native and Amphibious units.
Antigrav Base
You can build Antigrav Bases once
Commercial Antigravity tech is known.
Paradrop missions can be launched from that base.