I find that breaking things down really helped.
choose a country that appeals to you first- europe is pretty big so i guess its easy to get lost. pretend you're going on holiday and work from which place appeals to you most.
do you want to be able to get married on the same premises as dinner?- that will help narrow it down as i assume they will need a licence to perform that.
Also it will help to know what/who you need for catering. if one place can do it all then that's great but if you want to get married somewhere that doesn't do food then thats something else to think about.
have you thought about hiring an airbnb and caterers for the dinner?- can work out cheaper and more intimate. you can even hire castles on there!
do you need to think about cars and transport if you decide to do things in different places?
is it worth checking out local restaurants near to the actual marriage place- no cleaning up and great local food.
once you decided on the country, check out trip advisor for reviews
hope that helps a bit :-)