For  cheats in this game, use the DOS utility DEBUG. First backup
your game to be edited, then type the following from the DOS prompt:

ren fire.exe cheater.cht
debug cheater.cht
For invulnerability, type:
e 30cb eb 8
For infinite magic,
type:
e 201a 90 90 90 90 90 90
Then type:
w
q

Then rename CHEATER.CHT back to its previous name.