The windows are irrelevant you can hang some curtains or blinds, so from the outside they look normal, then cover them up with the proper sound material. A square room won't kill you either with the proper acoustic treatment. You need to figure out how many seats you want and need to look at height for seat riser. How tall is the ceiling. One thing I have learned on this site if you have a desire there is away. From the pictures it appears to be a very good space.