I agree the game would look more realistic if fortresses were not miniature versions of weirdly square strongholds that throw showers of arrows at a large range, but were instead larger concentric defenses, including patrols, palisades and killzones, then finally a fortified position on a pre-existing hill, where the garrison retreats when the threat is nearest.
But that would be hella complicate to realize and a miniature version of a conventionally shaped stronghold does the job of representing map control and also looks cuter in the process.