Constructed in 3000 BC, Knowth was extensively excavated over a 40-year period from 1962. Now reconstructed, the main mound contains two long passage tombs placed back to back. There are the remains of 19 smaller satellite mounds and more than 300 decorated stones in the main mound alone and represents the greatest concentration of megalithic art in Western Europe. The site was subsequently reused for defence and settlement purposes in the Iron Age, medieval and post-medieval periods. Access to Knowth is by guided tour only via the Brú na Bóinne Visitor Centre.