The Abbey of Saint Markovia was the first map I worked on that featured snow - it took some getting used to, and there were a lot of teething problems with regards making certain objects and pathways look like they'd been snowed on. Originally I just slapped the snow asset down on everything and tried to make it work, but I came back at the end and instead used a transparent layer of white through the Pattern Shape Tool. I've gotten a lot of mileage out of that thing.