When the creature affected by Tower Above attacks, you can target any creature on the battlefield with the triggered ability. However, if the creature you target can't block the creature affected by Tower Above (for example, because the attacking creature has flying and the other one doesn't, or because both creatures are controlled by the same player), the triggered ability does nothing. In that case, the creature you targeted is free to block whichever creature its controller chooses, or block no creatures at all.