The Trouble with Robots is a sort of single-player, sidescrolling trading-card game, with something of the feel of tower defense.
The backstory is that a flying saucer has landed in a fantasy world, disgorging innumerable robots. In a series of 20-something levels, you control armies composed of peasants, elves, centaurs, and the like against robots of diverse types.
Before a level begins, you select 5 cards from those available to you -- only a few choices are available at game start, but other cards are unlocked during play. There are multiple waves of attackers within a level; each wave, you are dealt new cards. Energy to play them recharges over time; you can use surplus energy to trigger lightning bolt attacks on individual robots, which means that, unlike so many tower defense games, you do still have things to do while attacks are ongoing.