Dragon Age can’t use the mount in haven

OK so, the first place I really started questing was In the hinterlands. While I was there one of the first things I did was do the farm quests and got the free mount, and I've been riding around happily ever since. But then I decided to check out haven properly. Anyway, when I'm in Haven and I open my radial menu, the horse icon is still there, but has gone dark and when I press it nothing happens! Then when I go back to the hinterlands it works normaly again. Is there a reason why I can't use my mount in Haven?