It seems easy. Tag rooms, doors, and other indoor features with level number (or floor or storey), put level selector to the map and show features just from selected level. End of story. But what if there are two buildings A and B connected by passage? And what if these buildings are on a slope and level A1 is on the same height as level B3? And what about mezzanines? Are stairs part of the lower floor or upper floor? And where to show it? Aren't some big lecture rooms stepped? And aren't they also used to take more levels? Masaryk University maintains geospatial database of its own buildings including polygon features like floors, rooms, doors, windows, or walls. It contains more than 200 buildings and 20,000 rooms. Based on the database we are building web maps in OpenLayers 3 for specialized users as well as for students and academic staff. Therefore we have faced similar questions as mentioned above many times and I would like to share our experience.