Depends exactly what you want Leslie.
Can be done in various ways, and my latest (unfinished) dungeon has a few examples.
If you want fading into blackness, Gambit has already posted.
If you want the walls simply to reach the top of the screen, then you can either take snapshots of each section of wall and add to it, or stretch the bitmap a little.
Stretching works okay with the DM2 Cave wallset, as you don't have to stretch too far to cover the ceiling. However, it doesn't look good when you do it with stone walls (for this, I took the DM2 stone wallset and added to it).
Also, Linflas' RTCWM program supports tall walls. It would be even quicker with this, but can become a little hazy.
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