In the mission, you must beat 25 special modified rounds on Medium Difficulty, using the two "Sockets of Power." Placing towers on them make them attack dramatically faster as well as having a slightly increased range and Camo detection.
On the 12th December 2018 update, The Crucible is added to BTD5 Mobile and BTD5 Steam as a special event. Players can play it once every 4 hours for special rewards that vary each time.
Placing a Tower in the Crucible with Overclock, M.I.B Call To Arms, Jungle Drums and T2 Specialties can make a Tower attack 5x faster, such that a 1/x Ninja Monkey attack nearly as fast as a Super Monkey.
If you put a 4-2 Spike Factory in one of the sockets, it appears that the hatch isn't moving, because of the hypersonic speed it is producing Spiked Mines at.
This is only true on Fast Forward, though it still opens and closes very fast.