You can use whatever mix you used to make the walls or mortar or concrete or Liquid Nails.
Why did you make 4 separate pieces? You could have used a 1 piece mold like a milk carton/jug, cardboard box lined with plastic, rigid foam insulation boards. Then you unmold (carefully) in 4 to 8 hours and carve or wait 24 hours and add pebbles/stones to the walls with mortar or hypertufa. Pieces of styrofoam can be cut and shaped for doors and windows then add the pieces as you build up the walls. Billie