It seems that buildings on hexes to which a creature could legally move (if the building was not there) appear attackable even if they are not (for example if one of your own creatures or a flying creature of your opponent's is on the hex it would need to stand on to attack. If you click on it, it acts like you are selecting a hex from which to attack, but there are no options presented. It does appear you can click it again to use the closest hex though. I'm never tried it myself, but during my last game the log showed that my opponent did a clearly impossible attack of this form. His creature dimmed but did not move and no damage was done.