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.