Muswell Models make cast buildings, primarily for model railways. The range of OO/4mm pillboxes interests me (there are even more in 2mm/N scale, but that scale doesn’t appeal as much to me). I have visions of a diorama at a crossroads with bored squaddies peeling spuds and stripping Vickers machine guns as they wait for an enemy that may never come.
20th century war
The spigot mortar (not strictly related, but interesting nonetheless.)
Pillboxes and defensive positions around Mount Bures in East Anglia