What are some of the best 3rd person Tower Defence games out there and what makes them fun?

I wanted to see what it would be like to defend your base alongside your towers, so I have recently dug up Sanctum 2 from my steam library and gave it a shot. The game felt a bit too easy to me and didn't age very well, but it did leave me wanting more. I have Orcs must die 3 on my list, but I was wondering if there were any hidden gems in that genre. I'm also curious to know what makes that kind of game fun for you.