You can find the High Gate Ruins here:
About half way there is a puzzle in a large room that has an elevated area. In front if you at the ceiling you will see four stones that have images on them, except one of them has fallen to the ground and is illegible. If you turn around above the entrance that you just walked through the symbols are shown again, this time all four animals can be identified. You will have to pull the levers in the right order. You don’t even need the second set of signs, you only have one possibility of the “unknown” leaver.
You can surprise Draugr in the Catacombs because they are resting in the alcoves. Sneak up to them and ht them with you sword for triple damage. There is a boss at the end, Vokun, who is somewhat tough to kill. He has a large quantity of health and, on the harder difficulties, does a lot of damage. Simply hit him with you r sword until you are low on health and then use a health potion to replenish that health. If you don’t want to use a potion you can maneuver around the large stone pillars and other cover that you can walk fully around so that Vokun can’t hit you and use magic to regenerate health or simply wait for the natural health regeneration. However the latter does take a long time and the use of potions or magic is essential unless you want to get very bored.