"Kingmaker" by Avalon Hill, published wayback in the 1980's.
Noble houses, mercenaries, alliances. Not too difficult in the mechanics department.
Finding copy might be difficult but not impossible.
Thanks for your reply but I'm already aware of "Kingmaker". It was a very good game I played a lot as a teenager back in the day. I'm tempted to try and pick it up on eBay but if possible I'd rather try something I haven't played before. It doesn't even need to be new as long as it's available somewhere. I was reading on BGG that "Empires of the Middle Ages" was re-released recently and it does sound interesting and is one I have never played but I was hoping to hear about some I've not heard much about.