Are you sure about that?
Yes, actually, I am-Castle Brians do NOT serve the same functions as fortified towns and castles, they are frequently located in remote an inaccessible areas, depend more on concealment than commanding key territory, and-this is important-
were not built by local lords but by the Hegemony to enable central rule. By the 2700s, they are either destroyed, hidden, and generally out of use across the Inner Sphere, and weren't built for positive-control anyways. In a Succession Wars era, they're lootboxes for PCs, and in the Clan Wars, they're active deathtraps if you haven't refit their systems totally because Clanners know all the secret entrances.
In real life, fortified towns and castles combined with the lack of formalized logistics, meant that armies could and did make incredibly deep penetrations of enemy territory that amounted to very little, while serious siege was the most difficult of arts because any army would deplete the locally available resources. Investing multiple fortresses at the same time invites defeat in detail or sallying out to skirmish and then retreat. But all this was only possible because the fortified towns and castles sheltered the wealth and economic centers of the countryside. Castles located on barren mountains or desolate forests where nobody would ever think to look for them were NOT the order of the day. You built a castle to hold down control over a region but the Brians don't do a very good job of that post Amaris, because they're either radioactive or practically invisible.