Hilfield Castle, also Hilfield Lodge, is a country estate and house about 2 miles (3.2 km) east of Watford and 1 mile (1.6 km) southwest of Aldenham, in Hertfordshire, on the outskirts of London in the United Kingdom. The estate contains two large reservoirs, one of which were built by French prisoners of war in the 1790s,. The grounds now form Aldenham Country Park. Elstree Aerodrome is located just to the east and the M1 motorway passes to the west.