The codewords...
Level  5:       RHRK
Level  9:       TBGY
Level 13:       XFCK
Level 17:       BKYZ
Level 21:       VXBB
Level 25:       XYLZ
Level 29:       XLZG
Level 33:       WJQK
Level 37:       BZVG
Level 41:       BRPK
Level 45:       BLHR
Credit level:   XWJR
Bonus level:    BCDF