how did you do the tiles? I think you done the entire floor in 1 detail tile? So it looks like a pattern. If you have done this you should fill or auto draw the groun for area's that size, detail tiles should be used for well detail :P
Sorry if im wrong, maybe its just the size and my eyes today.
As for the corner you will need to do some tweeking by deleting certain bits of the wall.. Which will then allow you to place the correct wall. However there is a corner bit for it i'm sure.