Northern Ireland is the "home" of the series. All interior sets are here in studios in Belfast - but there are also some great outdoor locations used. One of the most beautiful is Castle Ward in county Down. This 18th century house is surrounded by beautiful grassy areas that were home to a lot of the army camp scenes.
Just west of Castle Ward is Myra Castle, home to Dreadfort, the seat of House Bolton. Unfortunately the castle and parkland surrounding it are not open to the public, but you are able to see it from a distance when you're in the area.
The Clandenboye Estate, also county Down, doubled as Crasters Keep, but unfortunately you cannot see anything of it any more, as it was all burned down. The area, however, is very beautiful and has been used for a lot of forest scenes.
The Mourne Mountains, finally, are impressive by itself. It's a granite range on the coast of county Down and home to the highest mountains of Northern Ireland. In Game of Thrones, this location is used for Vaes Dothrak and the Dothraki Sea. The "Mother of Mountains" in the series is a sight to behold in this must-see area!