I have tried to add a sentry to my level 5 Tower, but the game acts as if something is already there.  I did have a level 4 Catapult on that Tower, but I destroyed it since I need at least one sentry to warn me of an attack.  I tried both a level 1 sentry and a level 5 sentry, but neither would place on the Tower.  I keep losing the sapphs I spend to buy these items since there is no place to put them.