Type in the codes in CAPITAL letters, an you find new option in the
 pause menu [= ESC].

 CHEATSEQ   Activate cheat mode
 HHROQTAT   Weapons
 TERHSRER   Next level
 CCHASERQ   Repairs
 HTARSREC   Items
 HERHSORE   Win level
 QHEROEST   Win all levels