Gather these parts first

Glue front and side panes into the base plate.

Glue the top rim onto the tops of the walls then glue the back plate on.

Glue the corner panels on as shown

Glue the front and side window bezels in as shown

Glue the optional interior door and frame inside as shown.  Feel free to place it anywhere on the back wall or leave off if you like.

Gather parts for the roof

Glue sides onto the roof accent panel and then glue the panel into the slots in the roof plate.

Gather up all of the roof struts

Glue each strut into the slots around the perimeter of the roof plate.

The roof should sit in the opening on top of the bunker now.  Do not glue the roof to the bunker so that it can be stacked and has a playable interior.