This is essentially a catch-all category for things that don't fit comfortably in other sections. Mystical Monuments are best treated as impassable terrain for the purposes of movement. They should be given a suitable Line of Sight value agreed upon by both players. Particularly large or complex Mystical Monuments might be made up of several terrain elements, such as hills or obstacles.
Below is a selection of different obstacles that you can choose to include in your games: