rebuilding Edinburgh Castle which became his principal seat of government. David's Tower was begun around 1367, and was incomplete when David died at the castle in 1371. It was completed by his successor, Robert II, in the 1370s. The tower stood on the site of the present Half Moon Battery and was connected by a section of curtain wall to the smaller Constable's Tower, a round tower built between 1375 and 1379 where the Portcullis Gate now stands. In the early 15th century, another English invasion, this time under Henry IV, reached Edinburgh Castle and began a siege, but eventually withdrew due to lack of supplies. From 1437, Sir William Crichton was Keeper of Edinburgh Castle, and soon after became Chancellor of Scotland. In an attempt to gain the regency of Scotland, Crichton sought to break the power of the Douglases, the principal noble family in the kingdom. The sixteen-year-old William Douglas, 6th Earl of Douglas, and his younger brother David were summoned to Edinburgh Castle in November 1440. After the so-called "Black Dinner" had taken place in David's Tower, both boys were summarily executed on trumped-up charges in the presence of the ten-year-old King James II (r.1437–1460). Douglas' supporters subsequently besieged the castle, inflicting damage. Construction continued throughout this period, with the area now known as Crown Square being laid out over vaults in the 1430s. Royal apartments were built, forming the nucleus of the later palace block, and a Great Hall was in existence by 1458. In 1464, access to the castle was improved when the current approach road up the north-east side of the rock was created to allow easier movement of the royal artillery train in and out of the area now known as the Upper Ward. In 1479, Alexander Stewart, Duke of Albany, was imprisoned in David's Tower for plotting against his brother, King James III (r.1460–1488). He escaped by getting his guards drunk, then lowering himself from a window on a rope. The duke fled to France, then England, where he allied himself with King Edward IV. In 1482, Albany marched into Scotland with Richard, Duke of Gloucester (later King Richard III), and an English army. James III was trapped in the castle from 22 July to 29 September 1482 until he successfully negotiated a settlement. During the 15th century the castle was increasingly used as an arsenal and armaments factory. The first known