I'm not a big fan of the tower defense genre, but PixelJunk Monsters has a unique appeal that kept me coming back for more.
The system of unlocking, building, and upgrading towers is simple, but not so much that it becomes boring. All necessary information is clearly conveyed to the player, which is very helpful. In addition to currencies and remaining villagers, you can see upgrade statuses of all towers, and their ranges. You can even see which type of enemy is attacking next, and how long you have until they do.
Unlike many tower defense games, enemies never attack your towers, they only go after your villagers. This makes the game less stressful than other tower defense games.
Towers get experience from kills and from you dancing under them. This experience will upgrade them over time, or you can pay gems to skip to the next upgrade level. Each tower starts at green, and upgrades to yellow, red, purple, and finally black.
The campaign is fairly long; it took me around 20 hours to complete all levels on the three main islands. Replayability is decent, with challenges, Steam achievements, higher difficulties, co-op mode, and a fourth island that includes randomly generated maps.
The downsides are the occasional difficulty spikes and the clunky tower selection spinner. The game also doesn't do a good job of explaining how towers are upgraded, which is why I explained it here.
Don't let the cute characters and pleasant music deceive you, this game can be tough.