Two pages and no one has mentioned:
Loads of palmerston follies round our way, some still in some sort of military use and others are council depot, library/community centre, garden centre, reclamation yard and so on…
Best kept one open to public (some open days and a few businesses including a bike/fitness coaching one) near us is Crownhill Fort:
Of course folklore fans and scientologists will love Tntagel Castle which is pretty lovely if not a lot of the actual castle left!
My favourite overall is Fort Libéria, in Villefranche-en-Conflent, Pyrenees. [/url]
It is sooooo cool, it even has an underground staircase all the way back down into the fortified town. And you can get to it on the coolest mountain railway EVER.