A large main tower with smaller towers nearby it, connected by tunnel-bridges near the tops. They're on islands of black rock on a lack of lava. A small rickety bridge connects the mainland to the main island.
At the top of each tower is an automated turret that locks on and shoots anything without a bio-signature clearance. There are windows in every tower, perfect places for gunmen to stealthily slide out the barrels of their guns and open fire.
The insides have everything needed to sustain three dozen people indefinitely, along with internet access and a secret escape tunnel that leads to a mobile spider-tank with laser beam cannons for eyes.
Yeah, my fort rocks!