Edinburgh’s busy High Street is now a popular spot for shopping and tourism, but a few centuries ago it was home to the United Kingdom’s most heinous prison, the Old Tolbooth. All that remains now is the Heart of Midlothian, which marked the point of no return, the entrance to the prison.

The Old Tolbooth was known throughout Great Britain as a vile place to end up. Though it began as a debtors prison, it soon imprisoned all kinds of thieves and murderers, as well as petty criminals. Some of these were women and small children. The Tolbooth was notorious for its instruments of torture—thumbscrews and pillories were common punishments. Executions were all public, performed on a platform above the town square. The most infamous criminals would have their heads impaled on spikes facing High Street as a warning to other would-be lawbreakers.

Its conditions were bad enough that in 1561, Mary, Queen of Scots ordered it torn down and rebuilt. The new building was modish and its new features included, among other things, a heart at its doorway. This didn’t indicate any kinder practices though, and the torture and executions at the New Tolbooth continued until it was demolished in 1817. The prison and its heart became famous through Sir Walter Scott’s The Heart of Midlothian, which fictionalized a riot at the Old Tolbooth.

The only thing left of the Old Tolbooth is the Heart of Midlothian. It is traditional for Edinburgers to spit on it as they walk past, though no one can definitively say why. Some say it’s a practice left over from when passersby would spit at the prison in solidarity with those inside. Others say it was the prisoners themselves who spat on the heart as they were released through the prison doors. Still others say it’s a gesture of good luck for the Edinburgh football team the “Hearts.” Nowadays, visitors are often told that spitting on the heart is the only way to assure you’ll return to Edinburgh again someday. Either way, watch your step when exploring the Royal Mile!