I liked it, especially the idea of keeping my towers from the previous level.
I'd say that the balance is a bit off, as I used only 4 different units after figuring out what each one did. (the ice spirit, the druid etc I found mostly useless)
I actually went ahead and played it at your site to try out the archangel, but i'd say it's not worth its price.
difficulty is way low, which makes it relaxing but slightly boring. I field a new farseer every level and group most units close to the center, which greatly reduces the need for new units when more of the map clears up. The strength of the defense grows faster than that of the offense, so by the last level there's hardly anything to do.
I enjoyed the game, though :]