The Circumfence is a sturdy wood-stakes-and-ropes barrier running almost a third of the Rim of the Disc, constructed by the island nation Krull. Hapless ships and vessels continually get washed toward the Edge, and by means of the Circumfence, Krullians are able to salvage many useful things; for example, many houses are constructed from parts of ships. In the old days, people who ran into the Circumfence would get captured by the Krullian navies and serve as slaves or even sacrificial victims (see The Colour of Magic). Tethis, a sea troll from another world, was one of the slaves who worked "on the edge".
Nowadays, Krullians rescue these people and charge them heavy salvage rates, a much more peaceful but just as profitable use of the barrier. On the other hand, the Circumfence gives rise to difficulties to people who attempt to launch a vessel into space by having it, to start with, washed over the Edge (see The Last Hero).