The walls surrounding Asgard were built by Hrimthurs, in return for which he was to receive in payment the hand of Freya in marriage and the sun and the moon. This was agreed provided that the work was completed within six months. In order to avoid the payment, Loki lured away Hrimthurs' magic horse, Svadilfari, by transforming himself into a mare and luring the horse away. The job was not completed on time and the gods evaded the payment.
Alternative: Blast