Passage: A section of rubble wall about 15 m (50 ft) long and 4 m (13 ft) high remains, incorporated into an outbuilding of the adjacent farm on its north side. The ruins and the later farmhouse are jointly grade II listed and the castle remains are designated as a scheduled monument. The area to the south of the wall has been excavated to reveal the ashlar bases of two polygonal towers. The site is on the east side of Castle Street in a private garden to which there is no public access.Some of the stone taken from the castle was used to construct the mid-18th-century grade II-listed buildings at 43 and 45 Castle Street, and other buildings known to have used the stone, but no longer extant, include old houses demolished to build the Castle Mill textile factory. The mill, now demolished, was said to have been built on castle foundations up to 4 m (12 ft) thick; 15 Castle Street also rests on the old foundation wall. It is likely that the former Melbourne Furnace and the Furnace Farm barn also used recycled castle material.An early 19th-century excavation found underground apartments "of considerable extent and superior workmanship", and excavations in the latter part of the same century found considerable foundations in the gardens of Castle Farm. Castle Mills housing estate contains a now-covered well 2 m (6 ft) in width and 15 m (50 ft) deep, and work in 1961 uncovered massive 5 m (16 ft) foundations east of the old mill and on the same alignment as the existing wall. Excavations in 1969–1971 found an extensive network of walls faced with ashlar, a door post, the base of a spiral staircase and evidence of an outer courtyard. Many stones had mason's marks. During construction works in 1988, masonry including the rubble centres of two large east–west walls was found in test trenches. Apart from the area of the turret bases next to the standing wall, none of the archaeology is now visible.

Question: When did the construction work find the rubble?
1988