Crannogs are man-made islands, usually located in marshy areas or small lakes. They could be accessed from the surrounding mainland by use of submerged causeways or small boats.
They were built for homesteads or used as cattle pens and safe houses. During the time of the Celts, cattle were the main unit of currency and therefore were very valuable. Cattle were often stolen in cattle raids and crannogs were one method of cattle protection.