Open-air assembly mound at St John's, from which the laws of the Isle of Man are proclaimed.

The hill stands in the Vale of St John, where the old road running north from Castletown crosses the road from Douglas to Peel. Slieau Whallian rises steeply to 978 feet on the south side of the valley. The Manx name is Cronk-y-Keeillown, the hill of St John's church, and the chapel of St John stands beside the mound.

A description printed in 1860 gives the dimensions: a grass-covered truncated cone about twelve feet high, 240 feet in circumference at the base and twenty-one at the summit, its slope cut into three circular terraces reached by a flight of wide steps on the eastern side, towards the chapel. The tradition recorded in the Manx Society volumes is that the mound was built of earth brought from every parish in the Island.

Promulgation from the hill is the reason it survived. A bill which had passed the Governor, Council and Keys was transmitted to the Lord Proprietor, who could reject it or give a general or qualified assent. When it came back approved, the Governor ordered a court on the Tynwald Hill, where the Act was read out in English and in Manx before the Governor, Council and Keys, and was then signed in St John's Chapel. Only at that point did it bind the inhabitants. The Duke of Atholl's assent of 5 June 1738, signed at Dunkeld, ordered the Act to be published and proclaimed on the Tynwald Hill according to ancient form and custom, and it was proclaimed at the Tynwald Court held on 24 June that year. The Act of Settlement and the Ecclesiastical Constitutions were proclaimed there on 6 June 1704.

The ceremony carried its own escort. Four horsemen from every parish, chosen from the most substantial inhabitants and led by the captain of the parish, attended yearly to bring the Governor and his officers to the hill on 5 July. The pathway from the chapel to the mound was strewn with green rushes.

The hill was also where sovereigns were proclaimed. William IV was proclaimed there on 5 July 1830, which one Manx Society editor records as the first known occasion of a sovereign being proclaimed at the place, and Victoria on 5 July 1837.

Antiquarian attention followed. Captain Grose drew the hill and made a plan of the ground in 1774, and his drawing was later used by the Rev. James Douglas in Nenia Britannica. About 1847, when the road at the west end of the mount was widened, a quadrangular stone cist was found, one of several burials recorded in the surrounding fields.

The 1417 assembly is less securely attested than it is often stated to be. Sacheverell and later writers record that Sir John Stanley, on his first visit that year, gathered the worthiest of the land to hear the Deemsters declare the ancient law, and promulgated it from the hill. The editor of Manx Society volume III rejected this, holding that there is no historical evidence for it and a high probability against it.