Solid tower defense game, with some major problems. Limited characters never getting proper reruns, and there being no way to guarantee a character you want on regular banners is pretty unacceptable for a modern gacha game. Newer content hasn't been great either, with two of the permanent game modes released in the past year (SSS and IS2) being heavily reliant on luck, and a good chunk of recent event bosses have had pretty bad, albeit not difficult, mechanics. Also has awful opportunity cost problems, especially when it comes to buffing units through modules.