This door is violently low res for such a huge thing. Also, it occurs multiple times in nearly every dungeon I've been to. I think my fix is satisfactory.

Drop the textures folder into C:\Program Files (x86)\Steam\SteamApps\common\skyrim\Data. Agree to merge folders and overwrite files. Unless someone already fixed this exact door and you downloaded it, you shouldn't be overwriting any image files.

To uninstall, remove these two files:
E:\Program Files (x86)\Steam\SteamApps\common\skyrim\Data\textures\dungeons\nordic\ruinsdoorextra01.dds
E:\Program Files (x86)\Steam\SteamApps\common\skyrim\Data\textures\dungeons\nordic\ruinsdoorextra01_n.dds



Note: Due to other engagements (and other games) I do not plan on updating this or any of my other mods, indefinitely. At a later date, my interest in Skyrim may come back but until then, please do not leave impatient comments about updates or fixes.