Some people are using sharp rocks, thorny vines, etc. to prevent slippage between bags. These things may help a little, but barbed wire does a lot more than prevent slippage –– it adds tensile strength to your structure. This point is often overlooked. Tensile strength restrains courses of domes from moving horizontally (expanding outwards from the weight above). Tensile strength also helps hold corners of buildings together. For instance, over time a corner could separate as the walls of a structure lean in different directions. But proper use of barbed wire between courses would help prevent this from happening.