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.