The best mobile tower defense game is buried inside Genshin Impact
Back in February of 2021, Genshin Impact added an event called Theater Mechanicus as part of its big Lantern Rite festivities. This mode was a sort of tower defense game that challenged players to build death traps of elemental towers against waves and waves of Hilichurls, Ruin Guards, Abyss Mages, and other common Genshin Impact foes.
The major twist with the event was that your character's power completely didn't matter. Whether you were a whale with a full bench of maxed out, S-tier, five-star adventurers or a free player with nothing of the sort, Theater Mechanics was an accessible, challenging, and super-satisfying event to take on in single-player or co-op. The only bummer about the whole thing was that Theater Mechanicus was a time-limited event and it disappeared after just a couple of weeks.