Ultima VII Cheat Room Location

Ultima 7 - Serpent Isle

Go to Stefano's old house in the woods(Moonshade) and get a crate from inside it.
Set the crate next to the stump right outside the door and climb on to it.
Walk around on the top of the stump and you will be teleported to an
islet with a teleporter, magic ax and a box full of guilders. 
If you walk NW into the water, there will be a secret passage leading north. 
Take the passage north and walk into the mountainside. 
You will find a vault (it is locked, but the key is at the building at the swamp right
before the Gorlab swamps, under the stairs) and many useful items.

Start Serpent Isle with:
"serpent pass"        - Serpent Isle
"serpent manimal"  - Serpent Isle with the Silver Seed

This enables cheat functions on the F-Keys and Alt-# keys.
The two most important are:

F2 - Cheat Menu
F3 - Teleport