For some reason, certain stone materials are the only substance that can penetrate force fields
Say you were able to develop a force field dome, the kind of device that you lay on the ground, and which projects a beam into the air, which then arcs down, producing a near-impenetrable hemispheric energy phenomenon that stops all but the strongest projectile, explosive, and directed energy weapons. A device say, that could even provide a degree of protection against strikes from our contemporary nuclear bombs.
This essentially resets you back to the pre-gunpowder era. Attackers can try to chuck stuff at your defences, but you as the defender have the huge advantage. You have two weaknesses, one old, one new. The old weakness is you can't easily resupply yourself, which allows your enemy to besiege you until you have no choice but to surrender. The new weakness is that your force field dome is just as impenetrable from the inside as it is from the outside, so you have no means whatsoever of counterattacking your enemy.
So let's handwave that there actually is a way to make little windows in your dome, and that is via some particular quality of the sort of blocks of stone that just so happened to be the kind of material that medieval people built their castles out of. So your fortification will be as follows:
- In the centre, the force field generator
- Surrounding the generator, all the various buildings and facilities needed to maintain your garrison.
- Right at the edge of the force field, stone walls, with protrusions pressing out into the force field, disrupting its effect just enough to create a small window through which a weapon can be pointed out.
The outside view of this fortification would be a stone structure, with tiered walls, stepping back in keeping with the invisible arc of the force field dome, dotted with crenel windows near the top of each wall level, through which a futuristic energy weapon peaks out.