The wall-builder story explains how Asgard gains a fortification and how Loki becomes the mother of Odin’s horse Sleipnir. Snorri tells it in Gylfaginning. The builder is an unnamed giant in disguise, not a documented mason from Norse history.
An impossible contract
A builder offers to complete a defensive wall in three seasons if he receives Freyja, the sun, and the moon. Loki advises the gods to accept on the condition that the work be finished in one winter without human help. The builder asks to use his stallion Svaðilfari.
The stallion’s labor
Svaðilfari hauls enormous stones and makes the project advance far faster than expected. As the deadline approaches, the gods blame Loki for the bargain and threaten him unless he prevents completion. Loki takes the form of a mare and draws the stallion away.
Broken terms and Sleipnir
Without Svaðilfari, the builder misses the deadline and reveals his giant nature. Thor kills him with Mjölnir rather than paying compensation. Loki later gives birth to the eight-legged Sleipnir. The tale exposes divine oath-breaking as well as giant threat, even though Snorri frames the builder as acting in deception.