Most promontories either are formed from a hard ridge of rock that has resisted the erosive forces that have removed the softer rock to the sides of it, or are the high ground that remains between two river valleys where they form a confluence.
Throughout history many forts and castles have been built on promontories because of their inherent defensibility. The promontory forts in Ireland are examples of this. Similarly, the ancient town of Ras Bar Balla in southern Somalia, which in the Middle Ages was part of the Ajuran Sultanate's domain, was built on a small promontory.
River confluences consistently provide an added defensive advantage to promontories, acting as a reliable natural moat for the enemy to overcome. The Citadel of Namur, a prime fortified location from the 10th century to this day, lies on the promontory at the confluence of the Meuse and Sambre rivers in the Walloon capital city of Namur, Belgium. Another good example of a confluence promontory fort is Fort Pitt, an English fort during the American Revolution that had previously belonged to the French as Fort Duquesne during the French and Indian War. The surrounding location is known as the city of Pittsburgh, Pennsylvania.
The dictionary definition of promontory at Wiktionary