https://ninjakiwi.com They have a number of free to play games and low priced games to purchase. Their bloons tower defense games are well made and regularly updated. They can be played for years while still progressing and developing.
I definitely played some regular bloons before they made it bloons TD. They have a lot of those TD games now.
I played monkey city way back when it first came out. It was browser based and still exists that way. They seem to beta test their app on their website. My nephew got into it and I started playing on the app. I had developed pretty well on the website as it was a downtime time killer for world of warcraft raiding and guild activities. So after years of not logging in to my we account I go back to it to play challenge and co op with my nephew and his eyes almost popped out of his head at how developed the city and subcity was. I have to say if you like tower defense games btd and monkey city are the best developed and continuously developed I have found out there. Their starting tutorials are easy to learn and their forums are extremely helpful with strategies and game issues. The btd thing was a variant from their bloons game which is sort of like angry birds. They have also made a fun sort of space invader style game based off the characters. They have a zombie based tower defense game and a fps zombie shooting game. All of the games are free with revenue generated from ads that can be killed off with minimal in game purchases. Their stand alone apps like btd are cheap as dirt and have huge content. I have to say they are the best game site I have found.
I worked on this game that may come out at some point: http://toucharcade.com/2016/03/31/boombot-announced-for-ios/
