An enclosure protected by a strong palisade, gatehouse and simple wooden buildings for the garrison. A full garrison normally consists of an officer (Lieutenant) and 20 troops.

A fort (2 Armories, 1 Bedroom, 2 Bunks, 1 Cell, 1 Common Room, 1 Courtyard, 3 Defensive Walls, 1 Dojo, 2 Fortifications (in the Common Room and Gatehouse), 1 Gatehouse, 1 Infirmary, 1 Kitchen, 1 Lavatory, 1 Office, 1 Storage, 1 War Room) can be used to establish your presence in a hostile environment, and provide a safe stopping place for travellers or adventurers. It might also be used to monitor potential threats in a frontier area, or to provide the basis for a wilderness settlement.

It costs 6,000gp in the borderlands and 9,000gp in the wilderness - this includes a single unit of Guards (Warrior-1)

Upgrades

The example shown (in the image) has been fitted with walkways along the walls (3xGuard Posts) which costs an extra 960gp in the borders and 1,440gp in the wilderness. However, this costs also includes recruiting another three units of guards (Warrior-1) and brings your garrison up to full strength.

It also has four light ballistae, and a unit of artillerists (Upgraded guard unit) to operate them. This upgrade costs 2,220gp.

You could fortify the outer wall to stone for 900gp in the Borderlands or 1350gp in the wilderness. This doesn't increase the height of the walls, however it sheathes them in stone and includes a shallow ditch around the outside.

You could upgrade two units of guards to Skirmishers, who would be able to patrol the area close by. This upgrade costs 350gp.

Employing an Officer (Lieutenant) would increase the effectiveness of your troops, giving them all +1 to hit and damage. However, this stops you using the fort as a home, because the officer will want the bedroom for their own use. Cost To be decided