Proposed Canon
It is described as being "a somewhat taller gray rectangle than those that comprised the rest of the Garrison." The Tower hosts the war room, as well as Overlord Firebaugh's quarters, private office, and public reception on the floor above it. The tower's top was manned with archers and ballistas for air defense, but poorly protected with air defense spells.
Delphie's claim may have only been 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".