HONGKONG'S new airport is being built on the site of an ancient fortress, archaeologists said yesterday. Artefacts recovered from Sha La Wan headland on Lantau Island point to a settlement more than 4,000 years old. The headland will be part of the Chek Lap Kok airport reclamation project. Excavations must be finished by the end of August for airport work to continue. Described by Antiquities and Monuments office archaeologist Chiu Siu-tsan as unique and a ''vitally important site'' Sha La Wan has convinced experts of the significance of investigating other headlands. A team led Dr Peter Drewett of the University of London began surveying the headland in late March and important discoveries prompted a second phase. Mr Chiu said the excavation had succeeded in retrieving a rich and significant amount of artefacts from the Late Neolithic period, about 2,500 BC. ''The most important finds are a series of about 15 post-holes which are likely to have come from prehistoric buildings. ''These post-holes are very significant because in the past only a limited number have been found and now we have the unique chance of getting a picture of the design of Neolithic buildings in the area,'' Mr Chiu said. He added that other relics from the site, including oven and burial remains, pointed to Sha La Wan being the site of a fortress or outpost used by an ancient community.