The Den is where the Smith family transform on the full moon. The Den is seen in 11 episodes of Wolfblood Series 1 and Series 2.
Within the Den, there is a false full moon, wood chips, hay, lamps, bowls and books about wolfbloods (e.g, that Romelus and Remus - the founders of Rome - were actually Wolfbloods, as was Genghis Khan) that Daniel Smith describes as 'books that can't be found in your school library'. Temporarily, a fake metal owl that "watches over all wolfbloods" installed by Shannon Kelly (with good intent) is present within the Den before being discovered and removed due to its hidden camera. Additionally, a secure, locked door is present within the Den to ensure when in wolf form can get out but there is also a hatch where at numerous cases (Leader of the pack andLone Wolf) when it is opened.
One wall of the Den is actually a bricked up passage leading to a vast network of tunnels once used by the Smith family hundreds of years ago. In Fall of the wild, Liam found the entrance to the tunnels at Aern Hollow, 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.