Complex sentence: A frequent characteristic is that the walls are galleried: with an open space between, the outer and inner wall skins are separate but tied together with linking stone slabs; these linking slabs may in some cases have served as steps to higher floors.
Split into simpler sentences: A frequent characteristic is that the walls are galleried (the outer and inner wall skins are separate but tied together with linking stone slabs, with an open space between). These linking slabs may in some cases have served as steps to higher floors.