1d4 alternatives to castles for when you need a fortification up on that hill.
1. Tax House
A fortified storehouse and bureaucratic center where taxes paid in local bulk goods (such as cloth, or salt) are collected, tallied, and made ready for transport. Could have a small garrison, or just a handful of sword/gun toting officials.
A sort of collective fortified granary, with individual chambers for different local families. Built as a citadel usually on elevated, rocky positions to protect surrounding farmland. Often used to store other valuable such as deeds, records, jewelry, clothes, carpets, and so on.
3. Signal Tower
Either optical semaphore or a classic pyre. Built high up with good lines of sight. Minimum garrison, and lightly fortified. Might double as a lookout and base for scouting parties. Of a similar vein, a relay or post station with stables of fresh horses and a cantina for tired messengers.
4. Monastery (abbey)
Big, isolated, and well-built religious compound. The monks likely own a significant amount of local land which they work themselves or rent to tenant farmers. Poorly defended with a lot of valuables.