The case opens on the boardwalk of Dufferin Terrace, where a rhyming riddle sends you to a wrought-iron shelter and its plaque. Under your feet, behind glass lenses set into the planks, is the corner of the house where Champlain died in 1635. Then you'll decode a question with its vowels crossed out and answer it from the Château Frontenac, which looks medieval but was finished in 1893.

On Place d'Armes, your photo of the Monument to Faith has a sticker over one detail, and only the real statue can fill it in. In New France this square was one of two where punishment was carried out in front of a crowd, and the crowd was the point.

A short way down Rue Saint-Louis, a low white house with a steep red roof has stood since 1675. Its heritage sign holds your next answer. Nothing went wrong here, for once: the restaurant inside is named after a novel by a man who lived in the house after three years in the city gaol for debt.

Behind the wall on Rue du Parloir, the Ursuline sisters have taught girls since 1639. You'll hold a photo of an engraved stone up against the real one and work out what's missing. In 1759 the nuns buried General Montcalm in a hole an English shell had made in their chapel floor, then kept his skull in a case for nearly two centuries.

Holy Trinity was the first Anglican cathedral built outside the British Isles, and it's home to Quebec's best-known ghost, a nun said to have been buried alive. You'll restore a word blacked out of a handwritten note using the text by the gate. Then the record shows why no nun could ever have been buried here.

The Morrin Centre is a library now, but for 55 years it was the city's gaol. You'll check a photo of its entrance against the real doorway and spot what's been taken out. Above that door hung the balcony where Charles Alarie was hanged twice in one morning, because the rope broke the first time.

In the City Hall gardens, four near-identical photos of the same sculpture each give it a different carved head, and only one is right. The ground under your feet went from Jesuit college to British parade ground to town hall, and now people come here to argue about parking tickets.

At the basilica, the app sends you round to a rounded tower to count the faces on the wall. Governor Frontenac lies in the crypt below with roughly 900 others. He had his heart sent to his wife in France, and she sent it back with a note saying it had never belonged to her.

From Montmorency Park on the cliff edge, where two parliament buildings burned beside cannons that never fired at anything, the route drops to the Lower Town. The giant painted mural covers a whole wall, and you'll find what has been painted out of your copy. Place Royale was the city's other punishment square. Jean Rattier was condemned to hang here, took the vacant hangman's job instead, and kept it for 23 years.

A few steps across the square, the little stone church of Notre-Dame-des-Victoires stands on the foundations of Champlain's Habitation of 1608. A scrambled message sends you looking for a year on the ground. Of the 28 men who spent that first winter here, 8 lived to see spring.

The file closes at the Breakneck Steps, the stair that has joined the upper and lower towns since the 1630s. You'll do one last spot-the-difference, on a candy shop's hanging sign, and everyone in the file is accounted for: the governor, the general, the prisoners and the hangman all used these steps.