The corner ruins are double skinned walls with detail on the outside and the inside.
The first step is to glue the outside walls to the base as shown below.
The next step is to glue the inside walls to the outside walls. Make sure you get the inside corner edge of the interior walls lined up with the outside walls.
Soem clamps are useful to hold them in position.
There are some extra bits that can be used to add some fallen bricks and planks to the base if required.
MDF Construction 