The heart of old Dublin occupies the ridge separating the Liffey and its now covered tributary, the Poddle. The Vikings settled on the south bank of the Liffey, where Wood Quay and the Dublin's Modern Civic Offices are to be found now. Excavations undertaken in the 70s and 80s revealed traces of 150 Viking buildings at 13 different levels, built between 920 and 1100. The castle was built on the southern flank of the ridge, overlooking the Poddle.