For the 2m wide walls you can set the grid the minimum size and clip a 4m wall halfway into its neighbor. There is way less Z-fighting than I feared. Or in some sets there are quarter round/ octagonal pillar pieces that have a square backside you can use. I know those are silly workarounds that do not fit in many cases. I also miss a piece to cover up the gap after a half archway. The thing doesn't go all the way into the ceiling, so the next wall piece either has to be another half arch in the mirrored direction or some weird custom ledge structure the game does not really cater for. It's ridiculous. The art team was clearly in a rush, assets are inconsistent, incomplete and the whole building system is just outdated and convoluted.