A crannog, sometimes rendered as 'crannóg' is an artificial island constructed, normally on a lake or any sheltered stretch of inland water, for the purposes of human habitation, and on which has been erected a defensive homestead of some type. They are extremely common in Scotland where there is hardly a loch that does not feature the remains of a crannog, but examples exist throughout Britain and Ireland, notably at Llangorse Lake within the borders of Brycheiniog. They are essentially a form of Celtic vernacular building, with the name crannog being derived from the Gaelic word crann for 'tree'; being naturally enough the predominant construction material.
Their use as a base for habitation stretches right back to the Neolithic Age and continues throughout history; there are references to their use in Scotland as late as the seventeenth century. The attraction of the crannog was based, of course on the defensive advantages of surrounding your abode by a large stetch of water (in the same way as medieval castles were often constructed with the added protection of a moat.) Crannogs were normally only accessible via a causeway and sometimes only by boat and therefore provided a secure and easily defensible home. Protecting yourself and your family from the threat of raids from hostile neighbours being a fairly constant consideration throughout most of history.
There are basically two types of crannog.
1. The solid base type
Which are essentially small islands, constructed either by taking advantage of a natural rock outcrop which was capable of being enlarged, or created anew by means of piling up quantities of stone or whatever other material came to hand on the lake or river bed.
2. The free standing type
Which consists of a raised structure, such as a stilt house constructed using vertical wooden piles on which a platform was then erected. This later type generally stood high above the water and was substantially taller than the solid base type.
Irrespective of the type crannogs are normally circular or oval in shape and vary in size, but are commonly between 15 and 30 metres in diameter. The superstructure is almost exclusively constructed in wood although there are
a few stone examples in Wales and the Hebrides.
Archeological interest in crannogs first began in the nineteenth century when it became the practice to drain Scottish lochs in order to create more agricultural land. This brought to light the artifical nature of many islands and revealed that what had been previously considered as a rather mundane mound of rubble was in fact the remains of an ancient dwelling.
Interest continues to this day as crannogs represent an important resource to archaeolgists, particularly those built in Scottish lochs. This is because Scottish lochs are both cold and relatively bacteria free which helps the preservation of organic materials. The excavation of crannogs can often reveals examples of seeds and plant materials that can be used to the diet of the inabitants as well providing plenty of material for carbon dating. Whereas the waters in Ireland don't provide quite the same advantages, the acidity of the water there does tend to 'pickle' the wood which also tends to assist in its preservation.
There is an example of a reconstructed crannog on Loch Tay in Perthshire, Scotland (see http://www.transcotland.com/crannog.htm) and another in Ireland at Craggaunowen near Kilmurry, in county Clare ( See http://www.stonepages.com/ireland/craggaunowen.html).