Using the plague theme for a tower defence game is a nice idea!

I do not know why but the music would make me SUPER AGGRESSIVE normally. In this case it calmed me down (it must be magic!). 

I played your game in a short YouTube video - feel free to check it out yourself:

neat and challenging tower defense game. It was pretty satisfying building a mesh of melee defenders though when I tried to start saving money to buy more ranged I kept losing so it seemed like my options were limited.

The art and music fit pretty well. If you decide to expand on this idea I think it could be interesting to have some variety to the levels (maybe tapered blood vessels?) or "boss" battles, or more defender types or additional special moves.

Nice job overall though.