The Rogue Den is a special military building released during the 2018 Fall Event. Rogues can be trained with this building. Training a Rogue does not cost any coins or supplies, but will take 24 hours. The first two slots to train a Rogue are free.
The Rogue Den is an upgrade of the Rogue Hideout and although smaller has exactly the same values.
To upgrade your Rogue Hideout, use the Rogue Hideout Shrink Kit item on the Rogue Hideout that is already situated in your city. Its upgraded version will then be placed directly on the same spot in your city. Recruitment slots previously unlocked in the Rogue Hideout will remain unlocked in the Rogue Den.