Only knights had full body armor, and they were basically aristocracy in that era. Rare class.

If you've seen World War Z in cinema, you know that breaching a castle wall is trivial for a zombie hoard. Just a matter of time really.

At best, the entire workforce of peasant farmers would be turned, and only the knights and kings would survive, if they can defend against the newly created zombie-peasant army.

Then they'd starve to death, because none of them know how to work a farm 😉