The castle stands on a rocky islet which due to drainage works and farming activities has now become linked to the shoreline through a curved earthen embankment.

[3] In July 1560 a treaty had called for peace and reconciliation; however, as stated, Lord Semple and others continued to commit many murders, stole cattle, burned down houses, set fire to crops and committed crimes even to the extent of "..not sparand to sla auld men of fowr skoir yeris off age, lyand decreppit in their beddis.".

[4] Although called to appear in front of the Justice-General to account for these actions he instead set about strengthening the garrison and the defences of Castle Semple and had also "..off new fortit ane hows within an ile in the loch of Lochquhinyeoch.

On 18 September Glencairn's musketeers or "hagbutters" approached the castle yards and Sempill's men shot at them with pistols called "dags" from the windows, not daring to go on the wall heads.

The young Earl of Arran joined the besiegers, with the English ambassador Thomas Randolph, and camped in a barn.

In the 18th and 19th centuries several bronze cannon with octagonal barrels were found in the loch around the peel tower, each carrying a falcon and the insignia 'IRS' for James V of Scotland.

[11] Castle Semple Loch was at one time around 4.5 miles long and nearly a mile wide; however, the loch was the subject of much drainage work between 1680 and 1774, resulting in around 400 acres of acres of rich land such as Aird's Meadow being made available for cultivation and grazing with a concomitant loss of surface area and depth of water.

[12] In William Roy's map of circa 1747–1755 the peel is shown on an irregularly shaped isthmus extending from the shoreline at Lochside House and the Black Cart Water shows signs of having been canalised.

Ornately carved stonework was present[16] and the fortification had the overall shape of an irregular pentagon with a sharp end pointed towards the head of the loch.

[17] As states the peel tower was built on an islet in the loch as shown by old maps; however, extensive drainage reduced the size and depth of the loch, and the creation of Airds Meadow involved the formation of an embankment that joins the islet to the shore on the Lochside House side.