Was considering what would be the best way to do this. LOS in Ultima 6 is weird since it seems to be location-based (once you open a door you can see the room inside), but you can see through walls (some times?).
I thought just replicating it for the sake of it didn’t make sense and would bring more complexity for map handling and editing. So instead I went for a full system with opaque tiles, so you just have to define the tile as opaque and the engine takes care of the rest.
The current implementation is a pretty simple raycasting, and a mask drawn over the map. It works almost great except the tweening of the mask moving is not in sync with the camera tweening, which causes some small artifacts in the mask border.