Arcane is a new element being added to YouTD.
Arcane is the counterpart to Iron, it is the element of magic itself. Arcane towers focus on mana, spells and mechanics relying on mana. Every Arcane tower is designed to be hungry for more mana and mana regeneration through powerful abilities with high costs and short cooldowns.
If you're worried about immune enemies, do not fret, as there are an assortment of towers that remove mana from enemies, thereby disabling immunity. In fact, many existing creep abilities have been reworked to also rely on a creep's mana, so stripping enemies of their mana is largely beneficial.