The urban structure of settlements on mountains and hills typically follow the “lay of the land”. Take for example the towns in the Baronia de Carapelle on the Abruzzo region of Italy. Built on the slopes of the Apennines, the architecture of these towns--Santo Stefano, Calascio, Castel del Monte--follow the contour lines of the mountain, with long roads slowly climbing up at shallow angles and stairs negotiating the changes of level at steeper angles.

The approach to Castelvecchio Calvisio, also part of the Baronia, appears to suggest a similar urban structure, with the newer architecture of the south-facing part of the town following the topography of the terrain and a zigzagging road with hairpin turns going up. But at the top, the medieval core of Castelvecchio reveals a completely unexpected geometry: an orthogonal arrangement of narrow streets within a tight oval perimeter. It has a main street running east-west following the long axis of the oval and perpendicular streets running north-south at small intervals. From above it looks as something like the fossil of some enormous turtle-like creature. Although uncompromising in plan, the straight geometry of the streets is allowed to curve from the center to the edge on top of the convex form of the land.

Where does the surprising geometry of Castelvecchio Calvision come from? That I don't know, but I'm sure that there is a fascinating story there...