Well-designed levels and mechanics that present good--and oftentimes harsh--challenges to players. Beyond the basic gameplay, the worldbuilding is nicely done, and the lore is quite unique. Despite the large number of characters (i.e. operators) in the roster, many of them are very memorable.

It's very doable to remain as a F2P player, since it's possible to clear most--if not all--of the levels with just the free operators.