Complex sentence: Constructed from local limestone in an "A frame" design, the walls are strong yet contain no mortar, although many have been neglected and allowed to disintegrate, replaced or contained by a mix of barbed wire and sheep fencing.
Split into simpler sentences: Constructed from local limestone and in an "A frame" design, the walls are strong yet contain no mortar. Unfortunately, years of neglect are allowing many walls to disintegrate, being replaced or contained by a mix of barbed wire and sheep fencing.