The Château de Vincennes is a royal palace in the shape of a tower: six storeys, with one great vaulted room on each floor. The keep has its own walled enclosure, and a second wall a kilometre round encloses that. The tower stands fifty metres high, counting the later work added on top, and no medieval keep left standing in Europe is taller. Charles V put it up in the 1360s in his family's hunting forest east of Paris, at a time when the city itself had become dangerous for kings.
Inside, you can walk up through the rooms he lived in: the great chamber, the study off it where he kept his books, and a chapel the size of a cupboard cut into the wall. No other French royal keep can be climbed like that. From the seventeenth century the tower was used as a prison, and the men shut in it scratched their names and their calendars into the stone. That writing is still there. A separate chapel in the courtyard, begun to hold relics and finished nearly two centuries later, holds Renaissance glass.