feature not usually found outside Ireland is the round tower, such as that at Clonmacnoise in County Offaly or the one on Devenish Island near Enniskillen in County Fermanagh. These were usually built within the monasteries that sprang up all over the island, as the country became the "land of saints and scholars". They were possibly defensive in nature, serving as lookout posts and a place of refuge during an attack (the door to such structures was usually quite high off the ground). Viking raids on Ireland's shores and monasteries were relatively common. St. Cronan's Church in Tuamgraney, County Clare, a pre-Romanesque church which dates from the 10th century, is the oldest church in continuous use in both Ireland and Great Britain. Eventually some Vikings settled permanently in Ireland, and the main cities were established by the Vikings. Although no buildings from that era are now intact, some street arrangements have their origins in the original Viking layouts. Remains of Viking dwellings have been discovered in many locations, but notably at Wood Quay in Dublin, King John's Castle in Limerick and near Waterford (where what is thought to be the original settlement at Waterford has been uncovered in 2004 during construction of the city's ring road). Medieval Ireland After early stone remains, the next most visible features in the Irish countryside are the innumerable castle remains, tower houses and intact castles. Apart from well-known and restored castles such as Bunratty Castle, many unknown remains (particularly of tower houses) exist next to newer farmhouses, or again, simply in fields. Carrickfergus Castle, built by John de Courcy in 1177, as his headquarters after his invasion of Ulster, is the most perfectly preserved Norman castle on the island. The castle at Cahir is also a particularly well-preserved example. Many fine churches in Ireland were also built during this time, such as St. Canice's Cathedral in Kilkenny and St. Mary's Cathedral in Limerick. Most common was the Romanesque style, as seen at Cormac's Chapel on the Rock of Cashel, and at Clonfert Cathedral in Galway. It was the Normans who brought the Gothic style to Ireland, with such buildings as Christ Church and St. Patrick's Cathedral in Dublin. Some of Ireland's main cities were built up and fortified before and during the mediaeval period. Limerick remained a walled city until the 18th century, while Derry's medieval walls still stand today. Such features as King