You can only place walls at a 45* degree angle as far as I know. There is a cheat that goes by "boolprop allow45degreeangleofrotation true" (no quotation marks though) that allows you to rotate some things to a 45* degree angle like furniture and such. Although you can turn the objects your sims may not be able to use that object. Use the <> keys to rotate objects as you place them. You may have to change true to false to place windows and doors in a diagonal wall though.