They consisted of cabins, blockhouses, and stockades.
A range of cabins often formed one side of a fort.

The walls on the outside were ten or twelve feet high with roofs sloping inward.

The blockhouses built at the angles of the fort projected two feet or so beyond the outer walls of the cabins and stockades, and were fitted with portholes for the watchers and the marksmen.

The entrance to the fort was a large folding gate of thick slabs.

It was always on the side nearest the spring.