Personally, I find things like an ordinary throne in a castle not very landmark-ey. A landmark should be something that really stands out and might even be a bit odd. It really depends on the universe you're creating, but try making things more unexpected and weird. Because if the player stops and thinks "What the hell is that?" or "Hey I wasn't expecting that here" then that will immediately become a landmark for them.
So if you were to have a throne, make sure it's in a big grand throne room which is much larger than all of the other rooms where the throne over-arches everything else and has high-contrasting colors.
Just my 2c