Defense Grid 1 & 2. Fun over the top tactical game. A lot of modes have multiple challenge modes once you've played through the map once.
Pixel Junk Monsters. Another top down. Good power difficulty ramp. Also map specific challenges.
DG 2 is a game I put a TON of time into on both PS4 and Xbone, love it to death and am still bummed the company went out of business before making a third one. It hit the thing I seem to search for in TD games really well, namely putting up machine gun towers and watching it chew through enemy hordes. Something about it itches a specific part of my brain that I really dig.
Pixel Junk Monsters is one of those games that I play a couple of levels of every time I turn on my old PS3. The art style and enemy types are just perfect and it strikes the right balance between player activity and simply watching the damage rack up. It feels less strategic and more like a puzzle game sometimes, but still a classic.