Jestertrek: I thought money for levels was random, too, until I figured out that it is directly related to # of kills and nothing more. I don't think you get anything for sending a guy through, but you get $5 for each kill (I think -- too lazy to test this again). Strangely, this does not really reward you for winning, but for killing, so in the early game it's best to rack up kills rather than slip guys past the defenses, as I did.
This is awesome, great job! I like your choices, e.g. not being able to sell units. Keeps the strategy interesting. Couple of things I would love to see, but that would not futz up the point of the game: a pause button; a speed-it-up button. I also miss the visible changes to the units as they upgraded, from defender.
I have to disagree with the spamming strategy. First of all, the later upgrades do a lot for you. For example, you eventually get a range upgrade (I think after 4 general upgrades). Secondly, if you just spam the level with novice mages, the ones near the exit are super weak in the endgame, so they are basically useless.
Seems to me best to start placement near the end of the maze, since XP will accumulate only for those that are landing attacks. Otherwise you get a lot of novice units that are fairly useless vs. stragglers.
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