I’ll tell you right away, we inherited Plastwood castle, so I know some things only from the words of the former owners. The castle itself consists of three parts, two sides and the front. There is no and never was a back part. So, if ypu want to close the gap, you need to either lean the castle against the wall, or buy another set, or make a wall from another constructor. Like from Magformers, for example, as we did in the photo below. Or you can simply bend the side walls towards each other and ignore to the gap between them (or patch it up with some tape).
