Well, there have been this Kingdom of Jerusalem, which is the only "civ" which was really founded by crusaders from 1099 till 1291. This would be the historical most accurat civ. However the flag wasn't exactly the flag of the crusaders. I don't know if it would be close enough for Monitor.
Kingdom of Jerusalem (Wikipedia)
There have been two more or less known kings in this Kingdom, Baldwin I and Henry II of Jerusalem. Special unit of course some crusader knights (which can build fortresses

), but I don't have a specific idea for a special building, maybe a place of worship beside of temples (monument, theatre, collosseum,...) or a millitaristic installation (city walls, barracks, horse stables,...).
The biggest problem would be the cities. I didn't look into the files but Jerusalem, Acre, Tripoli, Ascalon, Antioch might have been used by other civs allready, and there aren't many more known cities.