The issue with stacking that I came across is that there is a nice gab between them, enough to get your shoe into it. so if you can climb one you can climb all. you can grip the corner (there is a small gap between each container) and climb up pretty fast, and get your foot into the horizontal cracks. it's actually pretty fun to do. The challenge is much much harder (I couldn't do it) if the containers overhung even an inch.