The Prince's Lines are part of the fortifications of Gibraltar, situated on the lower slopes of the north-west face of the Rock of Gibraltar. They are located at a height of about 70 feet (21 m) on a natural ledge above the Queen's Lines, overlooking the landward entrance to Gibraltar, and run from a natural fault called the Orillon to a cliff at the southern end of the isthmus linking Gibraltar with Spain. The lines face out across the modern Laguna Estate, which stands on the site of the Inundation, an artificial lake created to obstruct landward access to Gibraltar. They were constructed to enfilade attackers approaching Gibraltar's Landport Front from the landward direction.

The Inundation was a flooded and fortified area of ground on the sandy isthmus between Spain and Gibraltar, created by the British in the 18th century to restrict access to the territory as part of the fortifications of Gibraltar. It was originally a marshy area known as the Morass at the far south-western end of the isthmus, occupying the area adjacent to the north-western flank of the Rock of Gibraltar. The Morass was dug out and expanded to create an artificial lake which was further obstructed by iron and wooden obstacles in the water. Two small fortifications on either side controlled access to Gibraltar. The only road to and from the town ran along a narrow causeway between the Inundation and the sea which was enfiladed by batteries mounted on the lower slopes of the Rock. The Inundation existed for about 200 years before it was infilled and built over after the Second World War.