Building work began in 1277 initially under Richard , who would later become Mayor of Chester in 1304. The castle and its earthworks were built by 1,800 laborers and masons using local Millstone Grit ashlar and sandstone. Savoyard master mason James of Saint George was assigned "ad ordiandum opera castorum ibidem" (to undertake the works of the same castle works (referring to all Welsh castle building projects)) in April 1278. November 1280 saw him beginning directly overseeing construction at Flint for Edward I as the initially very slow construction pace was accelerated. He remained at the castle for 17 months. James of Saint George then moved onto Rhuddlan to oversee its completion.