I think the tileset is missing a stone wall tile. For example in your animated screenshots, there are tiles to be able to do these type of S-pattern like these. 
The closest that I was able to find are these two tiles. Both of them don't work. It looks like I need some kind of hybrid between the bottom part of the tile to the right side and the top part of the left tile, but it is nowhere to be found in your tilesheet... 
Any idea?
Thank you so much for adding the missing tiles in less than 12h!! Amazing support!!