Tell me about medieval border fortresses, please!

[Dr. Strangemonkey]

In the D&D-Cosmos there are "improved engineers": Xorn, Earth Elementals, and so on. You have got to secure the very ground against that kind of borrowing dangers...
But I don't have the great Idea how to achieve this. A large area wall of force? Or a plate of steel laid in the floor? Something like this...

generally you fight engineers with other engineers and good detection equipment.

So I would say you would secure the ground with other earth elementals, xorn, and so on.

Bound outsiders are also a nice way to get punch at reduced upkeep.

 

[Nightfall]

Personally if I'm building myself a nice castle, two things I require is one made of LOTS of different metals, (so that if I can, I can make the walls absorb a LOT of stuff), and two, hire high ranking outsiders to do the constructions. *Contacts Demogorgon* "Alright in exchange for my soul, and the souls of my yet to be born children, I want the best damn castle in the cosmos. Sure I'll hold* (Mutters to self) "At least he was kind enough to not give me five different estimates."

 

[Elder-Basilisk]

If you're into House Rules, you could also allow a Lesser Forbiddance (preventing planar travel and teleportation but not inflicting damage) to be researched at slightly lower level and significantly lower cost.

If you use Monte Cook's Book of Eldritch Might, a larger area, permanent Teleport Redirect might work too. Teleporting characters all end up in a securely guarded "gatehouse." Friendlies are allowed into the castle. Hostiles are terminated with extreme prejudice (perhaps a crushing ceiling trap--although fifty archers with crossbows behind murder holes) might also do the trick.