From the outside, the Tugendhat House doesn’t look like one of the most important residential buildings of the 20th century: it’s just two white stucco cubes separated by an opening through which a few spiky treetops protrude. But as a tour guide led a group of 10 through this modern home in the Czech Republic’s second-largest city in early March, it was clear that there was much more to the house, the bulk of which is built on the steep hillside that drops away from the street.