Give each cell a defined entrance
For a prison cell in Malignus, start with a walled space and leave an opening for a Cell Door. A beta player described building separate cells and placing a door, a padlock, and shackles as part of the setup. Mossbomb’s official press kit also documents a captured adventurer held in a Cell Block.
Add the door, lock, and shackles
Place the Cell Door at the opening you left for the entrance. Add the padlock and place shackles inside the cell. Those are the three pieces described in the beta example. Set up one cell first so you can check that the door is on the intended entrance and the shackles are in the right enclosure before repeating the layout.
Keep the cell plan separate from the capture step
The cell layout gives you a place to hold prisoners. It doesn't tell you which command captures or moves a surviving adventurer. If you are planning for an invasion, build the holding space first, then check the prisoner controls available in your build when a survivor is ready to be captured.
FAQ
Which door was used in the beta example?
The player named a Cell Door when putting together the prison.
Where do shackles go?
Place them inside the enclosure you have set aside for holding a captive.
Does every dungeon door need a padlock?
The documented setup is for a prisoner cell. It doesn't establish a rule for every door or tell you who can pass a locked door by default.