sentences:  Outside the third ring of walls lies a defensive system of stone slabs, known as a cheval de frise, planted in an upright position in the ground and still largely well-preserved.These ruins also feature a huge rectangular stone slab, the function of which is unknown.Impressively large among prehistoric ruins, the outermost wall of "Dún Aonghasa" encloses an area of approximately 6 hectares (14 acres).