The Venomous Spire is the beginning of the long path to defeat the Plague Fiend. There are plenty of green explosive barrels and the area is guarded by Plague Grunts. The entire area is a ramp, spiraling up the airship that has been spreading toxin over the Forsaken Province.
Heroes arrive here after clearing the Haunted Cemetery. There is a Runestone at the base of the spiral tower, and it requires several switches to be hit. The Flame of Tarkana is hidden halfway up the spiral tower, and will only be obtainable once a variety of switches are hit, including one in a false wall just to the right of the spiral ramp. Reaching the exit at the top of the spiral leads to the Toxic Air Ship.
In Gauntlet: Dark Legacy this level is known as The Toxic Spire, and has been moved to the Sky Dominion. At the top of the spire near the level's exit is the Treasure Room, Cloud 9. Here the player can unlock the Unicorn.