Each unit is built using 6mm and 9mm Plywood. As these are made from plywood there may be some cosmetic marks on the ply but this doesn't affect the item.
The big corner already forms an ideal tunnel, but it can also be easily connected to the castle and the tunnel system. It can be set up both left and right. The corner is delivered in two parts and pushed together with plates in the middle. The connecting plates are already included.