The Den, is where the Smith family transform on the full moon. It is seen in 11 episodes of Wolfblood Series 1 and 2
In the Den, there is a false full moon, wood chips, straw or some kind and books about wolfbloods (e.g, that Romelus and Remus -the founders of Rome, were actaully Wolfbloods), and lights . There is also a locks and secure door so no one, when in wolf form can get out but there is also a hatch where at numerous cases (Leader of the pack and Lone Wolf ) when it is opened.
Near the Wolfblood Den there are mysterious tunnels, leading to an old Wolfblood lair. In Fall of the wild , Liam found the tunnels, and then went back after he recovered to get attacked by Maddy. On a full moon, the construction isn't very secure because it is old - as Liam managed to pull out a brick, and Rhydian and Daniel managed to knock one wall of the Den down too.
Daniel and Emma Smith are very persistent for Maddy to try out the Den (for ex: A Quiet Night In ) because the Den protects them from the wild and farmers. But the Den wasn't made by Daniel, in The Discovery , we learn that the Wolfblood pack have lived for hundreds of years.