The displays of heads upon the south Tower of London Bridge was one that had gone back centuries before Shakespeare comments on them. Indeed perhaps one of the earliest examples was William Wallace who had his head placed above London Bridge while his other body parts were displayed across the country.

The act of beheading seems to have been reserved for the most notorious of criminals- traitors. Within England the standard execution method was slow hanging.

If one was noble, you were afforded the rather quicker beheading. And if you were deemed traitorous to the state, you would be hung but only so far. After this the grisly ritual of ‘drawing’ and ‘quartering’ would begin (basically live eviscerating).

From most accounts the heads were placed on spikes above the south tower (chosen because the north tower didn’t command much of a view due to buildings in the way, but from the south tower the heads could be seen quite some distance down Long Southwark, the main thoroughfare to the Bridge).

On average they lasted around two to three weeks, before being dropped into the river below. More famed traitors could find their head dipped in tar to extend the time frame. It was not always just heads. Body parts of traitors were known to occasionally adorn the tower.

As for there being enough ‘traitors’? The show trials of Henry VIII towards the end of his reign, the religious passions of both Mary and Edward, and the machinations of the Elizabethan secret service was more than enough to keep the display ‘active’ shall we say.

Hope that helps.

Sources:

Salgādo, “The Elizabethan Underworld”

Engel “Lord High Executioner”

John Stow “A survey of London”