It goes back all the way to the Romans, not to the Medieval period - and was common across Europe right up until the French Revolution. Because the French royals had always ridden on the left, the French decided to swap to the other side because "republic!" Napoleon, who was left-handed and found it easier to fight from the right, spread the new system everywhere he conquered. America began driving on the right, too, because of the way the seats on their wagons were arranged. Britain never met Napoleon's army in the British Isles, so it kept to what was then the standard way - as still used by about 40% of the world's population.
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It goes back all the way to the Romans, not to the Medieval period - and was common across Europe right up until the French Revolution. Because the French royals had always ridden on the left, the French decided to swap to the other side because "republic!" Napoleon, who was left-handed and found it easier to fight from the right, spread the new system everywhere he conquered. America began driving on the right, too, because of the way the seats on their wagons were arranged. Britain never met Napoleon's army in the British Isles, so it kept to what was then the standard way - as still used by about 40% of the world's population.