All of the levels are beatable. The damage actually varies a lot more than that too, for example armour (which increases by a monster-specific value per round) is subtracted from damage, and then there's the "strong against" and "weak against" which either reduce or increase the damage again.
Flat-rate damage is kind of stupid in my opinion... if a tower hits for 25 and a monster has 150 hitpoints you know in advance that 2 towers spaced apart are all you'll need for that wave. That's not strategy that's being spoon fed.
Anyway the first upgrade of each tower significantly increases the minimum damage,so maybe you should factor that into your strategy.
spartyongarth - the tower icons came from Warcraft 3 and other graphics from Warcraft 2 as well. There's other TD games that use Warcraft's graphics too.
Deathcrunch - stealing them from where? Myself? How does that work exactly, when I'm not looking I copy & paste my files to somewhere else on my computer?
Guys, blue trolls are strong against arrow towers. You need to get cannon or elemental towers happening as well.
Remember you can view what a monster is strong and weak against in upcoming rounds by mousing over the round number up the top.
Are you using flash player version 8 blackbelt? That issue came up with someone the other day and it was because the older flash player isn't compatible.
About to upload a new version which has an 'easy' mode. Every level is beatable as it is now but it's not one of those games where it takes more effort to lose than win. ;)
Boos - not sure if Blizzard really mind, they allowed some people to make a whole open source version of warcraft 2, they just insisted the name not be too similar. They also don't seem to have done anything about all the other little web based games and stuff built on warcraft over the years.
Derringer came up with the plan for the AI, it's pretty sweet I think. With the bug you mention the towers are basically going to attack the easiest kill so I'm guessing the lower ones were the best option. The path the monsters are on and the distance from the tower aren't factors.
I've also made some more tweaks to help it run faster.
Ok. Towers now automatically choose the monster they are capable of killing in the least shots and they stay targetted until that monster's out of range.
Ok, the tower AI is on my to do list.
I've also uploaded a version where you can configure a lot of the game variables and help find the right balance if you'd like:
http://5minutesoff.com/thehorde/
Thanks CarlR and karilowns! Today I'm launching a second version that'll let people test out different values for the monster hitpoints, damage etc so we can sort out the last problems.
Few more minor changes - the rounds' bar up the top now only displays numbers, if you mouse over them you can see what monsters and their weaknesses / strengths / immunities. Also I made some non-visible changes that'll hopefully help a little more with making it run fast.
 "The Horde 1.0"){kind=icon}
 "The Horde"){kind=icon}