Overlooking the harbour is Jordan's Castle. Anciently it was the largest of Ardglass' castles and is named after the Jordan family who lived their for centuries.

The castle is the most developed version of the distinctive gatehouse type tower house which is unique to this area in County Down. Above the door are not one but two arched machicolations from where all sorts of things could be dropped on attackers. On top of the castle was built a dovecote where the pigeons were kept for game.

The castle is renowned for its stance against the forces of Hugh O’Neill who besieged the castle for three years from 1598 to 1601. The O’Neill sept fled when Lord Deputy Mountjoy arrived by boat from Dublin on the 17 June. They were pursued and a battle fought in Dunsford. For his efforts Simon Jordan the castle’s owner was awarded with a private bounty by Elizabeth I and a concordat – a treaty – recognising him as a Catholic Royalist. The castle was captured by the Irish in 1651 before falling again to Cromwellian forces. It was again captured and recaptured in 1689.

In the 18th Century the castle became vacant and fell into disrepair. When it was bought by Francis Joseph Bigger in 1911 it was both roofless and floorless. Frank, a lawyer, Gaelic revivalist and antiquarian, saved it from oblivion after a restoration of three months renaming it Castle Shane. The castle was opened as a museum for display of Frank’s artefacts, the walls were adorned with portraits of Ulster chieftains.

The castle was a hub for the Irish Revival and was often visited by Alice Stopford Green and Sir Roger Casement. It was here that an interesting event occurred in 1913. Frank recruited John McGinley and Charles Duggan who were working on the harbour to man Asgard for the Howth Gun Running. Many of these guns were used by the rebels in the 1916 rebellion. After Frank’s death in 1926 the castle was left to the government. For many years the building was open as a museum. It is currently managed as a scheduled monument.