After beating the true final boss, you'll unlock a series of four gauntlet-style special missions. They feature many more enemies than those found it the regular stages, combined layouts of certain stages, and lack checkpoints.

Mission	How to unlock
Special Mission 1	Beat the true final boss.
Special Mission 2	Beat the true final boss.
Special Mission 3	Beat the true final boss.
Special Mission 4	Beat the true final boss.