The handover of the city keys, Bayeux Tapestry, scene 27 / 11th cent.
Leaders and their men would swear not to fight each other if the other party "kept the peace," that is, if neither side broke the agreement. The two sides could also exchange hostages, i.e. members of the group who would be handed over in order to secure the agreement, with the understanding that if the "peace" was not kept, the other party would dispose of the hostages.
At first, Christianity did not profoundly change this conception of peace. Instead, peace eventually required that a leader (and his men) become Christians, since Christians would only trust other Christians. Even then, it didn't mean that peace couldn't be broken in the aftermath of the baptism.
In this scene from the Bayeux Tapestry, Conan, the Count of Britanny, gives the city's keys to William, the future King of England, as a way to surrender and make peace.