But no one said a perfect siege would mean 0 casualties, so no. I don't even know how you'd go about qualifying a perfect siege anyway, and I don't think that really matter in this context so...
Now, it's important to keep in mind that the vanguard was composed entirely of adventurers, some of them highly seasoned. They had a solid plan, were highly equipped and due to circumstances within the assaulted keep were able to get inside with -relatively- little loss. This is still D&D, where adventurers are much more powerful than many npc's. My barbarian took a direct hit, along with another character, by a cannon, but because he's a barbarian and has all those barbarian hp's, didn't even corpse. Does that mean it was easy? No. In fact, there were many points where it could have meant many more deaths.
I don't know what all you've learned about it ic yet, so I won't go too much in to detail, but there were cannons on the Foxes side which are a game changer when it comes to assaulting a keep, and as I said before-there were reasons the defenders weren't nearly so able to defend themselves.
It's hard to try and justify the outcome of an event to someone that wasn't there though, I think it'd help if you trusted the people running it and the people involved, and as someone else pointed out, keep your disbelief ic. Trust me when I say this-I've seen and been involved in much, much, much more unbelievable things here than that. Some of them I still can't wrap my head around.
Also, it's been said, there were more than 14 deaths.