Crannogs are ancient lake dwellings. They are found in Ireland and Scotland. Crannogs are circular in shape. Some are connected to shore by a causeway or pier; others are reachable only by boat. The oldest crannog yet discovered was built in 3000 BC! The first crannogs were stilt houses. They were built over a body of water and supported by long wooden poles. Crannogs built in medieval times were made of rock and erected on manmade islands. This kind of crannog is also called a roundhouse. And while no one knows why crannogs were raised on water, their location certainly made them easy to defend.