The Old Sandbox was a playable area that does not count toward the campaign. It featured as a testing course for a machine. The Old Sandbox has all of the objects, enemies, and NPCs, from Ipsilon, except the Marksmen/Archer enemies. It was removed in the Multiverse update.
There are 9 known hidden chickens serves as a hidden challenge scattered around the zone. Try to squish them all with your creations.