File:TRIM CASTLE - COUNTY MEATH (3138699003).jpg

English: The central three-story building, called a keep, donjon or great tower, is unique in its design, being of cruciform shape, with twenty corners. It was built in at least three stages, initially by Hugh de Lacy (c.1174) and then in 1196 and 1206 by Walter de Lacy. The keep was built on the site of a large ring work fortification that was burnt down in 1172 and rebuilt in 1173, following attacks by the Gaelic King of Connacht, Ruaidrí Ua Conchobair Rory O'Connor.

Much of the interior of the castle was archaeologically excavated by David Sweetman of OPW in the 1970s and more extensively by Alan Hayden in the 1990s. The surviving curtain walls are predominantly of two phases. The west and north sides of the enciente are defended by rectangular towers which date to the 1170s and 1180s while the other sides with its round towers dates to the very end of the twelfth century. There were two main gates into the castle. That at the west side dates to the 1170s and sits on top of s a demolished wooden gateway. The upper stories of the stone tower were later altered to a semi octagonal shape. A single round towered gate with an external barbican tower lies in the south wall and is known as the Dublin Gate. It dates from the 1190s.

Apart from the keep the main structures surviving in the castle consist of the following: an early 14th century three towered fore work defending the keep entrance and including stables within it which is accessed by a stone causeway crossing the partly in filled ditch of the earlier ringwork; a huge early fourteenth century three aisled great hall with an under croft beneath its east end opening via a water gate to the river; a huge defensive tower turned into a solar in the early fourteenth century at the northern angle of the castle; a smaller aisled hall added to the east end of the great hall in the fourteenth or fifteenth century; a building (possibly the mint) added to the east end of the latter hall; two fifteenth or sixteenth century stone buildings added inside the town gatehouse, 17th century buildings added to the end of the hall range and to the north side of the keep and a series of lime kilns, one dating from the late 12th century the remainder from the eighteenth and nineteenth centuries.