Been playing Arknights lately, its a newer tower defense style game using heroes (operators) instead of towers. It has gacha style card pull elements to get new operators, and a tier system from 1* to 6* ranking. Pretty fun and the content so far is easily doable with the operators they provide to you, so you don't need to rely on the gacha to progress.
Free to play with in app purchases of course. Worth watching some videos on to see if it interests you.