For invulnerability in this game, use the DOS utility DEBUG.
First backup your game to be edited, then type the following
from the dos prompt:
For the [CGA] version of this game, type:
ren e:
oller.exe cheater.cht
debug e:\cheater.cht
e 1e8d 90 90 90 90
e 31c7 75
w
q
For the [EGA] version of this game, type:
ren e:
oller.exe cheater.cht
debug e:\cheater.cht
e 1e62 90 90 90 90
w
q
Then rename CHEATER.CHT back to its previous name.