Kingdom Rush is definitely one of the most fun tower defense games I've played. There is a nice variety of tower upgrades and the enemies change up often enough to keep things interesting. Maps, tower placement, varied paths, rally points, and the characteristics special to different types of towers and units give the player the opportunity to outwit the enemy with tactics rather than just by overwhelming enemies with sheer power.
Unlike the mobile versions, the Steam port has no microtransactions. All the heroes and upgrades are unlocked as you progress through the campaign. It also looks like the developer releases new content and campaigns here every few months, which in addition to the multiple challenge modes on each map gives a huge amount of gameplay. Also, finding the easter eggs is a lot of fun, and it's clear from those and the achievements that someone's got a sense of humor.
This game wasn't even on my radar before I saw half my friends list pick it up during the summer sale, and I'm so glad I jumped on the bandwagon and grabbed it too.
Now if I could just walk around without humming the music in my head...FREEDOM!