The location used for the tower and forge is at an area of Glen Coe called The Study which is a viewpoint looking towards the mountains known as the Three Sisters. It is next to a track which is the old road through the glen.
At the west end of the track, just before it begins to go down to the main road through the glen, there is a stone cairn. This cairn is where the forge was built in the movie. Russel Mulcahey recounts in the Director's Commentary on the Highlander 10th edition Region 1 DVD that Queen Victoria once sat at this spot, hence the stone cairn. They couldn't use the cairn in the movie but weren't allowed to move it. However they did move it without telling anyone and put it back after filming.
The tower was built next to the cairn - only the bottom third of the tower was actually built, the top two thirds being special effects. The cottage was located back up the track just past the large bend. The winter shot of the cottage covered in snow was filmed in Summer with the cottage being sprayed in foam. The surrounding mountains in this shot being a matte painting.