A: There's only one way to win a tower defense, build in labiyrinth.
I always wanted a map to have at least HINTS to help new people makeing long paths... (to not being screwed up in the third wave)
B: Add a hint to also help new people to NOTICE the useful [ready[ button. If the user is not doing anything, "talk" to him.
C: You don't have to use air units, just give them the ability to pass over structures with collision size 0.
D: The Collossus strike it's cool, but them are paralized from the center to the head, I don't know what did you do to make them "cliff off" spikes, but they're not moving ther torso at all. (very ugly)
E: You should add a table of the defeated/income(1or2) waves, also give them special abilities like: fast, single massive hp, long group, immune freeze, immune "spike/lurker", "air", etc... It's fun to think how to defeat the next wave.
F: You should improve the spine's model while the player upgrades it... Likes morphing to spore and changing theyir model, or size, color, light, addons (in the center) of the original spine.
G: I think its funniest to create spikes from an egg toss (like the infected marines from infestor).
H: As already said, improve the difficulty ;)
Of course, if people plays it and users reviews it, you have done a good job. But keep moving! Make your map grows ;)