Arknights is a gacha tower defence that was originally made for mobile clients (in fact, it’s only getting official PC client later this month).

In terms of gameplay, I enjoy that most levels are short by the standards of tower defence, so I can get 2-3 new stage clears in a 30 minute session, usually (provided I’ve actually figured out the strategy). There is some passive progression but it won’t give you all the upgrades you need and there is space for optimization and decision making in resource management.

While gacha monetisation model is an understandable turn-off for a lot of people, the difficulty balance allows even fully free-to-play accounts to keep up with the levels and the rates are the most player friendly in my experience.