I found a similar game to this, called Master of Defense. You can play online or download the Windows version here. The download is a trial version but I bought it because it's an awesome game!

P.S. I made it to level 81 in Tower Defense. The trick is to use only plant towers. Put them in line with the three horizontal paths on the right. You will have 3, 4 and 4 towers. You may lose some citizens in the early levels, but once you upgrade the towers sufficiently you will get very far easily.

I want to see a screen shot of where your towers are when you get to level 81. I did get to level 77 using only green towers (and one blue to catch the ones that got away from the greens early on) and I tried putting 3 on the top row, 4 on the middle and 4 on the bottom row... Didn't quite make it that far, though.

I want to see a screen shot of where your towers are when you get to level 81. I did get to level 77 using only green towers (and one blue to catch the ones that got away from the greens early on) and I tried putting 3 on the top row, 4 on the middle and 4 on the bottom row... Didn't quite make it that far, though.

Unfortunately I don't have that screenshot But I made a screenshot of the placement and which towers to upgrade first.



The highlighted towers should be upgraded to level 4 before all other towers are upgraded so that they have sufficient range. After that, start upgrading all other towers gradually, including the highlighted towers. You may lose 5 or 6 citizens before wave 20, but the remaining citizens will last a long time.

I feel pretty good about managing to get to a score of 72 myself.