Stone Tower guards the Great Pass, the only practical invasion route for a large army into Fortunata. Usually invasion means that somebody has united the Hussars and they are carrying out a massive raid for loot and slaves.

So it's not really a Tower. It's garrisoned by the Stone Guard, a passionate volunteer militia with a self-assigned mission to protect Fortunata.

Most states of Fortunata are bound by custom to provide a yearly donation. However, logistics being what they are, food and basic supplies come from the dependent agricultural communities around the Tower.