The line properties are probably my main issue as well. If you don't follow everything very closely and remember things very accurately you might try to erase a line and nothing happens...this is caused by two lines in the same place. Sometimes you'll lose the wall (only one line there) and sometimes you'll erase only part of the line (due to clicking more than two spots to make the original line). The most important thing to pay attention to is the white/blue colors...if you see some diagonal funky blue lines and shapes on a white wall then you've got more than two lines in the same place.

As for the building, I think it looks great