I have a smallish castle in a fantasy world modeled pretty closely after the 13th century (Edit: there are significant cultural/religious differences. The basic living conditions, level of technology and methods of warfare are the same). The castle is basically a mott-and-bailey redone in stone instead of wood. It's in hill country, with mountains not too far away.
The lord, Simon, was recently called to court. (He's a war hero, and the king wanted him there to add prestige, honor, etc.) His wife, Signy, is left behind to tend the estate along with their young daughter, Lia. (Their son Laury is off squiring for another lord.) It's summertime, and this part of the realm has been at peace for twelve years. There is a road to the capital city, where court is being held, and while it's for the most part safe (as far as 13th century roads were safe, anyways), there are places with outlaws etc.
My questions are thus.
1) What kind of force would Simon have at the castle while everyone was home?
2) How many of these guards/soldiers would he take with him for a journey of this sort? How many would he leave behind with Signy?
3) How large a force would it take to capture the castle with the number of men-at-arms he left with Signy? The attack is a complete surprise, and they manage to lay hands on Signy before anyone in the castle really knows what's going on.
4) Is this set of circumstances believable? It's very important to the story that Signy be in the castle while Simon is gone, although Lia could conceivably go with her father. It's up in the air right now if Signy is with child; if she would ordinarily go with Simon, this might explain why she would be at home.
Edit: Some more about the circumstances of the attack.
Edit to this edit: There was very bad wording on my part here. Let me rephrase.
One of the men in the attacking force (who is disguised as something NOT an enemy soldier-- I haven't decided exactly as what yet) begs for shelter from the castle (before Signy knew that there was a force coming at her). Signy lets him in. He stays overnight, and during that night the attacking force sneaks up to the castle. Come morning, the man begs leave to thank Signy, she goes to accept his thanks, and he puts a dagger to her throat and uses her as a tool to force the guards to open the gates. The attacking force comes in and slaughters Signy's guard.
About Simon: as I said above, he's a war hero. He was also born a commoner. He managed to sneak a message into a besieged city during a rebellion, allowing the forces in the city and the army coming to relieve the city to coordinate their attack and beat the besiegers into a bloody pulp. That battle was the turning point in the war, and the rebellion failed. The King-fellow, whose throne had been saved, decided to reward Simon with a gift of land-- the person whose land it used to be had sided on the wrong side of the rebellion and had his head removed. I don't know exactly how big the lands are.
Edit 2: What sort of title would Simon have, with the situation above? I doubt that he has much power in the realm of politics, or that his land is of great strategical importance.