100 acres is such a small amount of land (695 yards x 695 yards, or less than a sixth of a square mile) that it can be easily occupied, and the two entrances -- by definition close to each other -- sealed from the outside. Even worse, when you're in a cave, you've surrendered the high ground.

Now, to the answer... there must be a reason your kingdom is down there instead of on the surface like every other kingdom, and hasn't already been conquered: you're mining a mineral that's valuable to your neutral neighbors.

Because it's valuable to your much larger neighbors, there's a tacit agreement between them and you that they'll protect you from each other and outside enemies in exchange for selling them this valuable mineral at very favorable terms. IOW, you're buying protection.

100 acres is such a small amount of land (695 yards x 695 yards, or less than a sixth of a square mile EDIT: the Vatican -- smallest country in the world -- is larger, at 109 acres) that it can be easily occupied, and the two entrances -- by definition close to each other -- sealed from the outside. Even worse, when you're in a cave, you've surrendered the high ground.

Now, to the answer... there must be a reason your kingdom is down there instead of on the surface like every other kingdom, and hasn't already been conquered: you're mining a mineral that's valuable to your neutral neighbors.

Because it's valuable to your much larger neighbors, there's a tacit agreement between them and you that they'll protect you from each other and outside enemies in exchange for selling them this valuable mineral at very favorable terms. IOW, you're buying protection.