There, they could be exempted from the normal prosecution which, in those days, was quite severe (see, for example, The Law's Hall of Horrors).
But the ordeal, even within sanctuary, was no piece of cake.
The fugitive had to remain within the walls of the sanctuary, abandon his or her oath to the king, followed which they had a short period of time to leave the country.
They were considered to be "dead", so much so that their land was forfeited to the King and their wife considered to be a widow.
If they refused to renounce their oath, they could be starved out of the sanctuary.
Henry VIII of England even took to branding them with a hot iron before they left the country just in case they tried to return; they could then be quickly spotted and arrested.
Abolished from the common law in 1624 and, in France, at the time of the Revolution, the principle of sanctuary continues today, in somewhat altered form, as diplomatic immunity under international law.