Yeah, Roman permanent legionary forts were more organizational than defensive. Too strong for a raiding party or small strike force to overwhelm even when undermanned, but not really meant for the business of being sieged. They were there to keep the troops in one place, secured from ambush, and able to receive, supply, and train their reinforcements. It was much more important for everything to be NEAT and ORDERLY so the men (and supply trains) could get around where they needed to in the fort with minimal fuss than planning to frustrate attackers.

Whenever a good Roman fort survived the chaos of the Empire’s fall, though, there’s very little reason for the locals to not keep using it. All that stonework, put up by a polity that regarded 5,000 full-time troops as a ‘basic’ military force, in a time when a few hundred troops withdrawn from field work could be the total defense of an imperial capital? Much better than the equivalent woodwork fortifications possible with the local labor, even if those Romans ruins are in poor maintenance!

When dedicated castles of the High and Late Medieval Period began to be built, though, they were specifically made for the tempo of siege and counter-siege that characterized medieval warfare, and so looked very different, and typically were much harder to actually penetrate and overcome. Wouldn’t want to organize 5,000 men inside those walls year-round, though… between rubbing shoulders with everyone inside and the non-intuitive layout, I imagine soldierly tempers would be very short!