Is there a functional reason for wizards to live in towers so often?
D&D (& most modern high fantasy) was heavily influenced by LOTR. Saruman lived in Isengard (a tower), therefore the common modern high fantasy trope is that wizards live in towers.Here’s a thread explaining why Saruman lived in the tower of Isengard. 
https://www.reddit.com/r/AskScienceFiction/comments/33b4v3/l...