In 1878 Manuel Vicens Montaner commissioned Antoni Gaudí, just out of architecture school, with the design of a summer residence in Gràcia, then an independent municipality.
The stunning house is no longer on the outskirts of Barcelona -- it's on a narrow street in a densely populated area now -- and is privately owned and not open to the public. It's also pretty far out, close to Parc Güell, so I don't think gets many visitors, which is a pity because it's really worth seeing.
The door is from this spectacular building, with the same palm heart and marigold fence as at Parc Güell: