No, it's not. The "nice imposing castle" is not a type of fortification you can build, it's a region type called "stronghold".
And yet a level 3 fortification is a castle. Castles are hardly a rarity in the medieval setting BM is modelled after and many of them stand well away from cities in regions of questionable strategic importance because wealthy magnates were keen on imposing stone walls.
I've always thought a stronghold would be more like the imposing roman legionary base at Carlisle, the ruins of which were still being marvelled over in the 8th and 9th centuries. That's far beyond the concept of "nice imposing castle".
