Various historical accounts place your typical medieval 'army' (what you really had was a collection of lord, barons, and landowners, each with their own retainer of men) as having anywhere between 3:1, and 10:1 archers to men-at-arms, depending on when in the medieval period you look at.
Let's say for the sake of argument you're looking at 5:1. Your typical medieval man-at-arms uses a billhook, a light padded-jack, and possibly a helmet/crude gauntlets. Bills are good, they keep the zombies at range, they're wedged so that they have pushing power, and a block of billmen would move and fight as one. An average block would contain 200-300 men, usually formed in 3 arrays (cavalry formation), and you have 10 of these. That's a lot of pointy things. Behind your bill lines are 8000 archers. Let's be clear, your bill lines exist purely to stop your archers getting rushed.
Archers are cheap, arrows are easy to manufacture in bulk, and they can kill things without risk of dying themselves. 523
Each archer has at an extremely conservative estimate (looking at the travelling armies of the Battle of Towton) 6-10 arrows, and by law has been training weekly with his bow since the age of 7. At the very least each man can hit 5 zombies with his arrows. He then puts down his bow, unsheathes his shortsword and buckler, and joins the bill line. Realistically as a defending force, each archer is likely to have access to near unlimited numbers of arrows. Medieval armies were very, VERY good at killing other humanoids that tried to take their things.
Now your enemy have only once chance of beating them, and that's to have more/better archers, and your question states they don't have any at all. Now <PERSON> puts the number of troops normally needed to conquer a defending force at 10:1. Take into account the attacking force is an untrained horde, and you'd be looking at forces greater than 100:1 before your 10000 strong band of merry men are even remotely worried.
TL;DR, your zombies haven't got a chance in hell. 523