If I leave the mask editor and answer the save question with 'yes' the file is not saved (extension .prop). Then I saved it with the save icon as .prop but the format is not text and its size is about 23 MB. When I reloaded and set collision in the menu to on I could not see a difference regarding collision. The building was turned 90 degrees to the right. I assume prop is not the appropriate extension. I am near by to give up my attempts.