I have 2 questions about using multiple levels.

I have created a base level (main floor) and 3 other levels FrontPorch, Furniture1 and Furniture2 all at the same elevation. When I click on the levels above the main window the base level is visible and I can add items in the other levels.
But when I create a Level4 ( a second floor) all my other levels disappear and I can not use the base level as a guide.
What am I doing wrong?

Also, is it possible to merge levels?

Thanks very much,
Tarpon