On the long stretch of the backyard's east side, we built a leveled enclosure for Shannon's "play chips".  Play chips are Cedar wood chips from lumber mills chopped up into small little pieces ... most parks around here have them under their playground equipment to cushion kids when they fall off swings, etc.  Shannon's new "Castle Playset" showed up the following week.  I ended up rounding the corners of the enclosure with a router and staining it with a redwood stain to match the playset.