Level codes:

MLEYS - level 1 (brutal)
RLEYS - level 1 (normal)
FLEYS - level 1 (easy)
ZPWTR - level 2 (brutal)
APWTR - level 2 (normal)
HPWTR - level 2 (easy)
XQWXT - level 3 (brutal)
IQWXT - level 3 (normal)
KQWXT - level 3 (easy)
PMPEM - level 4 (brutal)
MMPEM - level 4 (normal)
YMPEM - level 4 (easy)
UOENC - level 5 (brutal)
FOENC - level 5 (normal)
COENC - level 5 (easy)
ONSSW - level 6 (brutal)
KNSSW - level 6 (normal)
NNSSW - level 6 (easy)
IZEPO - level 7 (brutal)
LZEPO - level 7 (normal)
PZEPO - level 7 (easy)
EJUGL - level 8 (brutal)
WJUGL - level 8 (normal)
OJUGL - level 8 (easy)
TYUPQ - level 9 (brutal)
EYUPQ - level 9 (normal)
WYUPQ - level 9 (easy)
MITFD - level 10 (brutal)
LITFD - level 10 (normal)
QITFD - level 10 (easy)
QDOGN - level 11 (brutal)
KDOGN - level 11 (normal)
GDOGN - level 11 (easy)
RAMPL - level 12 (brutal)
XAMPL - level 12 (normal)
HAMPL - level 12 (easy)
GMLSN - level 13 (brutal)
CMLSN - level 13 (normal)
SMLSN - level 13 (easy)
JPNIS - level 14 (brutal)
YPNIS - level 14 (normal)
DPNIS - level 14 (easy)
BBRBT - level 15 (brutal)
ZBRBT - level 15 (normal)
YBRBT - level 15 (easy)

DR0KN - secret code to watch the end movie without actually beating the game.