Fort Hammenheil, Portuguese-Dutch fort on a small rocky island at Jaffna Lagoon entrance, Sri Lanka.

Fort Hammenhiel is a fort built on a small sandy islet in the narrow channel between Kayts and Karaitivu islands, guarding the sea route into Jaffna Lagoon. Its plan is an eight-sided polygon with the gate in the southern wall and a triangular projection on the northeastern wall. Two small buildings stood inside from the Dutch period until the early 20th century.

The Portuguese built the fort in the mid-17th century of quarried coral, naming it Fortaleza do Caes, to guard the entrance to the Jaffna peninsula. In March 1658 the Dutch, under Captains Cornelies Reb, Piester Waset and N. van der Reede, captured it and renamed it Hammenhiel. They rebuilt it in 1680, adding a stone breakwater, filling the hollow ramparts, replacing the upper floor with a stone vault and building a brick-lined reservoir.

The Dutch name Hammenhiel means "Heel of the Ham": they thought Ceylon's shape resembled a smoked ham and the fort sat where the shank bone projects. The Dutch kept a garrison of about thirty soldiers under a lieutenant or ensign, and the fort held nine large dungeons for gunpowder. Today the Sri Lanka Navy runs it as a tourist hotel.

You reach the island by boat from the mainland, and sea conditions can change depending on the weather. Wear comfortable shoes since the ground is uneven, with rocky and narrow paths throughout the site.

A large water reservoir on the north side was built by the Dutch using special bricks and still holds fresh water today. This system for storing drinking water is several centuries old and demonstrates the practical engineering knowledge of that era.