I just put in some dates, they can be flexible and it populates airfare. It's actually dynamic so will search as you move. I've also heard good things about kayak but google is my favorite. It brings up main cities and smaller ones that sometimes have better prices.
Staying in a castle would be awesome!! So, on google flights - can I just put in Europe and it will pick the cheapest airfare at different airports? I have never used it before. That would be most helpful.