The most important criteria is that characters have to be able to be based in either King’s Landing or Sunspear for the long term. Furthermore, characters have to be able to fit into either court.
This means that virtually all characters need to be nobles. Smallfolk and merchants are only possible if they have a position or occupation that allows them to be at court and interacting with the nobles. Bear in mind that the Seven Kingdoms is a highly stratified society. Maesters, septons and septas are possible but generally discouraged unless you are a very experienced MUSHer who is able to handle finding roleplay in such a role.
For King’s Landing, characters should generally be from one of the Seven Kingdoms except the North, the Iron Islands and Dorne. Characters from the North are approved occasionally and characters from the Iron Islands are approved quite rarely. For Dorne characters should be from Dorne.
Foreign characters are generally not available as PCs. Experienced MUSHers who have read all the books may occasionally be approved to play a foreign character, but generally only after playing on the game with a Westerosi character first and generally only for time-limited plots.
Female warriors and any kind of magic users are highly restricted concepts, only occasionally available to experienced MUSHers with good knowledge of the setting who have been playing on the game for a while.