I've played a few rounds with td and here are my suggestions. It's an unorganized collection, because it's already late at night and I can't wait till tomorrow ;)

1.
It's pretty hard to balance all the towers and creeps, because the tactic varies with the amount of players. In a 2 player match you get less spammed with creeps than in a 4 player match, and thus the cannon and boom tower are quite useless with low players.

1.1.
The placing prize for the boomtower should be higher. During the matched with td I've fighured out that it is the best to place one of them and upgrade it to level 3. Further upgrading doesn't worth, and during the first waves (or however the time between the income is called) I wasn't even able to spend all my money on creeps, bacuse the boom tower bashed the first three creeptypes even without upgrades.

1.2.
Teleporters are really cool and effective to save your bacon, but I haven't tested if it's so effective to upgrade them.

1.3.
At the current prize / damage ratio the rapid tower is pretty useless.

1.4.
The Volcano was the only tower that felt really well balanced so far.

1.5.
I like that the freeze tower gains and looses some of it's range during the upgrades. I've usually upgraded it to lovel 3 (where it slown 'good' and has a range of 2). I haven't upgraded it further, but judging from the regular BTG I shoiuld do so in the future.

1.6.
I haven't used the cannon and surge tower, so I can't say anything now.

2.
The towers from level 4 and higher are too expensive in general. It doesn't worth to have a few highgrade towers, instead it's better to have lots of grade 3 and 4 towers.

4.
I'm missing a 'tutorial' for the symbols next to the creeps. I know that the two green arrows that form a cycle mean 'backwards', but I don't know where the difference between the running man and the winged boot is.

5.
I'd like to see some kind of sandbox-mode.

Thanks, I appreciate all your comments. once I get more feedback I'll compile them all together and make any needed changes.

Hopefully in a week or so I'll release a way to better get people into games. :)

I could report my experiences with mp but after i have read njulian´s post, i realized, that all my thoughts are pretty subjektiv and therefore maybe useless.
I m thinking very different about most points.
For example the best tower for me is the rapid tower and i thought it is too cheap.
I thought boom tower is by far too ineffective and expensive.
I prefer fewer towers with higher levels.
But the crucial point is that i have not found a good strategy how to play multiplayer and therefore i cant give sparticus valuable inputs for balancing.
During the game i m always thinking, that i m doing really stupid.
I m confident that i can improve and write something more helpful in the future after much more mp-games.