It is described as being "a somewhat taller gray rectangle than those that comprised the rest of the Garrison." Tower's top was manned with archers and ballistas for air defense, but poorly protected with air defense spells.
According to Delphie Temple the Tower offers very insignificant bonus for spell casting.
Delphie's claim may only be an excuse for choosing the Dungeon as her quarters and refusing to protect the Garrison with air defense spells.
Real World References
The name is a pun on "minotaur".