A promontory fort is like a treehouse, but on top of a big hill! You know how when you climb up a tree, you can see everything around you? It's kinda like that, but with a fort on top of a hill.
People a long, long time ago built these forts to protect themselves from enemies. They would make walls and ditches around their fort to make it harder for someone to attack them. They could see everything that was going on around them because they were up high on the hill.
It was also a good place to store food and water because it was easier to guard from up high.
So just imagine you and your friends building a big fort up on a hill so you can see everything that's happening around you and keep all your toys and snacks safe. That's what a promontory fort is like!