The arrows came from inside of the tower, through the open door.
I probably should have given you the size is my desciption...my bad. The tower is about 70 feet in diameter (5' thick walls, so 65' interior diameter). It is intact up to about 25' high, where the upper floors are rubble.
FWIW, though, the Temple proper makes the tower look small by comparison.
I've been doing all of the rolls myself, for the most part. For combat, at least, I'd like to stick to that format. However, if you are making a skill check or something such as that, I am fine with you using invisible castle to make a roll. Or I will do it myself, either way.