.geometry = {
            primitive: 'box',
            height: opts.height,
            width: opts.width,
            depth: opts.depth
          };

          delete opts.height;
          delete opts.width;
          delete opts.depth;
          delete opts.idx;

          Object.keys(opts).forEach(function (key) {
            cube.setAttribute(key, opts[key]);
          });

          return cube;
        }

        function addTower (opts, numCubes) {
          opts = opts || {};
          if (!('widt