Great gameplay. Plan on getting the full version. But promise me something. Once you collect enough revenue, hire a professional graphic artist to overhaul the graphics. It's not bad as it is now but it could be so much more in the right hands. Maybe now that you have a hit on your hands you can attract some major talent. You should check out PixelJoint, you can find many great pixel artists there.
Tip to the developers. You might want to use the Flash API's ACTIVATE and DEACTIVATE events to implement an auto-pause feature whenever a user clicks outside the game or whenever the game loses focus.
@Gjinko: Custom maps are out of the question. Unlike most tower defense games, the maps are not generated from a collection of tiles, instead each level map is a unique rendering done by the artist on which path data is overlaid. (check the Ironhide games forums for more info).
While not the first to integrate RPG elements into a TD game this one has certainly pushed it the farthest (thus far). Still, to reiterate other comments, the resource management system takes away from the experience. Maybe reducing the dimensions of the grid to 6x6 may help. You should check out "Kingdom Rush". The developers' of that game mixed-in a lot of RPG elements into that game without making too much of an impact on the core TD mechanics. You might be able to draw a few pointers from how they implemented barracks and such.
However, when all is said and done, this is a great achievement (even more so for a 48-hour comp).
@wolfbill: I think the "use-more-mana-as-you-upgrade" mechanic makes sense. After all, the more powerful a spell the more mana it should use. However, if there were some special item you could equip on a spell to have it use less mana then I could see how that could work.
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