This paper examines the construction and design of a 7-km long embankment, probably built for King Jayavarman IV between 928 and 941 CE, as part of a new capital. We calculate that the capacities of the outlets were too small, and conclude that the embankment failed, probably within a decade of construction, so that the benefits of the reservoir stored by the embankment and the access road on top of it were lessened substantially. We explain how the design was sub-optimal for construction, and that while the layout had a high aesthetic impact, the processes for ensuring structural integrity were poor. Simple and inexpensive steps to secure the weir were not undertaken. We speculate that this early failure may have contributed to the decision to return the royal court and the capital of the Khmer Empire to the Angkor region, marking a critically important juncture in regional history.