At this time, Sendai was written as 千代 (which literally means "a thousand generations"), because a temple with a thousand Buddha statues (千体, sentai) used to be located in Aobayama. Masamune changed the kanji to 仙臺, which later became 仙台 (which literally means "hermit on a platform"). The kanji was taken from a Chinese poem that praised a palace created by the Emperor Wen of Han China, comparing it to a mythical palace in the Kunlun Mountains. It is said that Masamune chose this kanji so that the castle would prosper as long as a mountain inhabited by an immortal hermit.
Turns out that the city's name has a pretty cool back story after all. Learning gets.
BTW on the kanji above, 体 is a counter for humanoid forms, such as statues. 臺 is just the old form of 台.
* Magical hermit post is so the next blogging craze.