Redoubt
“A reinforced, highly defensible structures built into the natural terrain known as Redoubts are often constructed to take advantage of locations such as cave entrances, mountain passes, or river fords. Communities may build Redoubts as defensive positions in times of war when their farms and towns can’t be defended effectively. Lords trying to tame and control newly claimed lands often make the construction of a Redoubt their first order of business as a temporary measure until a keep or larger fortification can be built.”
The example shown, to the right, is built around a large animal lair, which would have needed clearing first :} The large tree has a Guard Point set in the branches and extra boulders have been brought in to make a rough defensive wall, then a druid was called in to speed-grow bushes and briars around it. Most of the other 'rooms' are given space in the main cave, which has water and a narrow, defensible, entrance.
The Redoubt isn't obvious to raiders, both the inner and outer entrances are concealed and difficult to get into. It provides a good, initial, base in the wilderness and can serve as an outpost for scouts later in the game.
It costs 5150gp to build a Redoubt in the Borderlands or the Wilderness. It is one of the few buildings that do not attract a wilderness premium. The Redoubt comes with a unit of five experienced Security Guards. However, you probably want to upgrade those, and add another unit, of Scout/Skirmishers (450gp) to round out the garrison.
Upgrades to Redoubts are difficult, and need careful thought so that they do not disturb the hidden nature of this small stronghold.
Note: This is just an example, to give you an idea of what you will get. Any redoubt you build is likely to have a different layout, as it will need to conform to the landscape in the area.