Add a debug mode (compare the existing ones e.g. for showing polygon nodes or drawing buildings half- transparently) that filters on level tag values. Key combinations should change the filter, e.g. Ctrl+1 could toggle level 1, Ctrl+2 toggle level 2 and so on.

Once level filtering is activated by the user, relevant render items like buildings, placemarks, roads should react to it. E.g. a placemark of a shop that has level=1 tag should not be rendered anymore when level 1 is not included.

The existing debug modes should be a good inspiration, make sure to play around with each of them and look into their implementation to see how they work.