A TD is a tower defense. Creeps line up in a turkey shoot, and you mow em down or lose. Some loop around twice, some you can maze. Mine is a one shot walk by, no maze.
Whats hard? Hmmmm...
30 Tweaked creeps. Not too bad. Names, armor type, hit points, speed, bounty, etc.
5 Races. 10 Upgradeable towers. Thats 100 towers or so to make.
Graphics, projectile art, attack speeds, special abilities, tool tips, upgrade dependencies, gold cost and damage.
Play each one 20 damn times to balance out gold costs, damage, and attack types.
8 Player lanes, 5 movement regions each, thats 40 triggers for handling movements. One huge trigger with arrays would fire a couple hundred times, and... you don't want that.
Custom triggers, abilities, and variables to do neat cool things other people can't.
Difficulty, diaglog boxes, player conditions, forces, and triggers to handle em all.
Making an 8 lane map beautifull.
Making it ALL balanced together, and not suck.