I HAVE HEARD of a chimerical castle in Umbria that surveyors have tried to appraise in vain. Approaching it from the main road, you pass under a yawning portcullis. This leads to a central garden with a bubbling fountain and a decapitated statue of its first resident, one of several condottieri under the employ of Braccio Fortebraccio. A thick wall girds the castle, its gate protected by two north-facing crenellated towers that pierce through a slowly lifting swathe of fog. The walls may have endured numerous sieges and earthquakes, but only snakes, bats and hoopoes have survived the harshness of this deserted Eden.

Take the path round the eastern wall, and some clever trick of perspective makes the façade disappear entirely. The castle assumes a different character. The walls are less medieval, and therefore less skillfully fortified. Many parts lie in bombed-out shambles. The surrounding hills are drier, less green. Even the temperature seems several degrees warmer. Paved roads crisscross the landscape. An unseen train chugs along the banks of a previously unnoticed tributary nearby.

Walk to the back of the castle and you find yourself on a seemingly different latitude. The air is dense, moist, unhealthy. The grounds are wilder, more lush, ringed with hostile forts and remnants of catapults, trebuchets, and battering rams.

Rounding the castle you discover an even more remarkable discrepancy. The walls are draped with rose bushes. Pomegranate trees flourish in the blinding sun. Richly embroidered silk curtains frame the windows. The air is filled with the scent of Indian spices.

Coming back to the entrance you realize you have reached another castle altogether: the gardens have been rearranged, the statue is that of a gryphon, the towers face east, not north.

In your confusion you might try to skirt the castle once more, but all you find are still more previously unnoticed details: on this side a subterranean Etruscan well, on the next a secret chapel containing the finger bones of Saint Christopher, and on yet another the skeletal remains of inhabitants cannibalized during the Black Death.

Trace your way back and you are even more lost than before. For no two sides remain constant, and every shift in perspective transforms not just the castle, but the landscape, the fauna, the weather itself.

You could leave the castle, of course, from where you came; the same gate leads back to the main road. But wherever you return, though still familiar, is no longer the same: for the castle has shown you that there is no fixed space, that the world rearranges itself with each moment, that each vantage point is a reawakened universe, and you, helpless against this current, are likewise altered completely.