HELLHOUSE needs YOUR own doom2.wad (Doom II game data from id Software -
we can't legally include it).

EASIEST: just double-click "PLAY HELLHOUSE.bat" - it tries to find your
doom2.wad automatically (Steam / GOG) and copy it in for you.

If it can't find it, put your doom2.wad in THIS folder (next to
PLAY HELLHOUSE.bat), then run it again.

Where to get doom2.wad if you own Doom II:
  Steam: ...\steamapps\common\Doom 2\base\DOOM2.WAD
  GOG:   your DOOM II install folder
Don't own it? Buy Doom II on Steam or GOG (cheap, often on sale).
