I have always subscribed to the conceit that North and South London are mirror-worlds separated by the River Thames. So that for each and every place, person, and thing above the river, there is a corresponding item below. There’s someone just like you, a stranger living a life linked to your own in many mysterious ways, south of the Thames. Maybe you share one another’s dreams. By this definition, London lies flat like a two-dimensional map. Either North London is South London’s Heaven, or South London is North London’s underworld. In any event, it is a perilous crossing, regardless of whether it is done on foot, by bicycle, car, bus, or Underground, via bridge, tunnel, or ferry. When you cross that muddy, navigable, sacred body of water, be certain to make an offering to the three River Sisters, or hang on closely to your traveling soul.

One example of such magically twinned and interconnected places: Alexandra Palace, far away on a hill overlooking London in Harringay, and Crystal Palace, its opposite number and ghostly double. If London were a planet unto itself, you could draw a line of fourteen miles between these two Palaces, and it would serve almost precisely as a slightly titled axis—one that eerily intersects with the M25 Ring Road encircling the city—for strange magnetic forces. Each Palace has a television/radio transmission tower, being the highest spot for miles around. Both Palaces are not and never were palaces, not for royalty, they were People’s Palaces, follies in the mold of the Eiffel Tower and the Iberian-American Exposition, a fake Venice in Seville.

But of course there is no Crystal Palace, the place burned down in 1936, leaving its stone Sphinxes to molder into the underbrush, and a tiny corner of the intricate plate-glass structure left as an example of what the site might have looked like. So Crystal Palace is a vision of what Alexandra Palace might have become if it had been left to fall to ruins. Or Alexandra Palace Park, with its rose garden and fountain, duck pond, deer park, ice rink, skate park, and planned BBC television museum, is a vision of what Crystal Palace might look like today if it had survived. You cannot go to Crystal Palace and not think about rebuilding, or about the vagaries and chances of historical accidents that spare some places and destroy others. Sphinxes patrol both palaces, but it is not clear what their riddle is.

It’s silly to mention it, but when I went to Crystal Palace I kept thinking that a Crystal Palace ought to have a Crystal Skull buried somewhere in a cornerstone, a talismanic protective charm. The day was a cold one in February and what the place really is, is a set of grand staircases leading to nothing, a bleak winter overlook with remarkable sunsets. I found the bus station behind the park to be the most fascinating thing about the place. I like to explore how the infrastructure of the city fits together. I enjoy visiting places that are missing, destroyed, overlooked, or passed over. I found a little alcove in the broken stone of the back of a Sphinx, where people had shoved their drinks bottles and crisps packaging.

It had to be done: I tore out a page from my notebook and wrote something about the next clue on the treasure hunt being located at the Sphinxes in the Palm Court inside Alexandra Palace. Then I had to figure out what to say in the note I would place up there in North London in a secret place, hoping to please a stranger that might retrace my own journey between the two palaces that, for me, make up the polar ice caps of the alien planet London in winter.