2 1/2 might be generous. sick artwork tho. level by level:

Level 1: Attack ✓ ok
Level 2: Canyon ✓ ok
Level 3: The Catwalk ✓ eh
Level 4: The Combine ✓ eh
Level 5: The Fistula ✓ eh
Level 6: The Garrison ✓ bleh
Level 7: Titan Manor ✓ 😐
Level 8: Paradox ✓ good
Level 9: Subspace ✓ ugh
Level 10: Subterra ✓ mehhh
Level 11: Trapped on Titan ✓ UGH
Level 12: Virgil's Lead ✓ good
Level 13: Minos' Judgment ✓ good
Level 14: Bloodsea Keep ✓ ok-ish
Level 15: Mephisto's Maosoleum ✓ fuck off
Level 16: Nessus ✓ decent
Level 17: Geryon: ✓ pretty good
Level 18: Vesperas: ✓ pretty good
Level 19: Black Tower ✓ decent/annoying/bugs
Level 20: The Express Elevator to Hell ✓ eh
Level 21 (secret level): Bad Dream ✓ lame