Scottish history--need time period suggestions.

I know next to nothing about Scottish history. My story is set in the far north of Scotland, and I'm hoping for a sort of medieval time-frame, but I don't know much about the time periods surrounding this era in Scotland.

One of my MC's must go off to war at some point for roughly a year, and I would like there to be a sort of overlap between indigenous religion and the spread of Christianity, so that there might be a priest and a local "witch" worshiping the old gods at the same time.

Other than that I'm fairly flexible, I'd just like something that works to center my research around. Thanks to anyone who offers assistance!

Thanks, guys! I'll def. check out these links and see about YouTubing the documentary. I suppose since it is fantasy, I could be quite vague with my timelines and even the actual location. I've chosen Gaelic names for the characters, but other than that there is very little that requires me staying in a certain time period.