The Sluice Tower.

The Sluice Tower was a large structure on the outskirts of Undertown, overlooking the Edge itself. It was a draining pipe for the Undertown sewers. During floods, the gates of the tower were opened to allow water to flow out of the sewers.

In Clash of the Sky Galleons, Wind Jackal, Quint, and Menisculis visited the Sluice Tower in search of Turbot Smeal. Menisculis detected and killed a waif assassin. An enormous flood of water then swept all of them out of the tower and over the Edge. Quint and Wind Jackal were rescued, but Menisculis tumbled into the void.