Here's how the doors work.
This is one of the 5 x 5 rooms.
The centre panel in each 5 square section of wall slides out.
A door can be slotted into the gap.
The doors have a v-shaped grove on their vertical edges.
The vertical edges of the gaps in the walls also have a v-shaped grove cut into them. When they are being made I glue a length of cotton bud into the groove which dovetails neatly into the groove of the doors. The wall pieces also have a v-shaped groove.
With the walls being made from 5mm thick foamboard it is very easy to make the v-shaped groove during construction. Just make sure the glue is dry before putting wall back in place before the plaster wash and paint coat.
When designing the dungeon each 5 square section of corridor wall or room wall has a lift out section which can be a wall, or a 'secret door' or a real door. The 'secret doors' can only be revealed under certain conditions.