The trouble with this card is that it's very easy to use Bricks (and not altogether difficult to use Recruits either) to push one's Tower past 50. Unliving decks with Titan cards can easily raise Magic and end up with a large number of unused gems, all the while raising defenses. There isn't much that can be done to prevent its use.
I think it would be interesting if this card affected both (or, perhaps, all) Towers. That way, building your own Tower would serve as a deterrent. Then again, I'm not sure what would happen if it's played anyway since I don't believe it's currently possible to have both players reach 100+ tower through one card played. Would the game, as currently coded, recognize a Draw like it does when both towers are destroyed in one play?
Perhaps it should only work if both (or all) Towers are 49 or lower? Or maybe you gain +(Own Tower - Enemy's Tower), so 60 vs. 40 would give +20?