The werewolves in the movie were designed by Bob Keen of Image FX. The same team who created the werewolves in Dog Soldiers (2002) which is also set in the Scottish highlands but was not filmed in Scotland.
The castle in which Kelly Ann and Lee find the abandoned baby is actually a real castle in Mugdock Country Park near Glasgow, Scotland where a large portion of the film was shot. The exterior of the castle is exactly how it looks in the film, however due to old age, the interior of the castle was crumbling and deemed unsafe for the crew to work in. The interior of the castle was a set built inside an unused factory in Anniesland, Glasgow and most of the set, including the spiral staircase leading to the castle tower was made of polystyrene which was then painted by the art department to resemble real stone.