Haltherrion
First Post
To me, a castle is a fortified structure a local warlord or feudal lord uses as his base of power, or as a means of projecting his power over an area. As such it relies on two elements: it must be a military fortress, and it must be focal point for government power. I don't think its status as a residence is even as important as the fact that it is a seat of government of some sort. Generally speaking, fortresses stop being castles when the seats of government are moved to fancy unfortified buildings in urban centers.
In short a castle is a fortress where authority resides, particularly authority primarily borne from the military power embodied by the castle itself.
Would this include something like the Khe Sanh fortifications with various command posts including the battalion commander's CP? Or WWI Verdun if the fort contained a commander controlling the fortification forces? The forces in the region beyond the fort?