I struggled between choosing this episode and one from season two. The fact I choose this episode, directly following the one before it, is sheer coincidence.
The reason I chose this episode is because it shows stark contrast in the Merlin/Morgana relationship considering what we now know of their friendship due to the events at the end of Season/Series 2. Furthermore, this episode reveals just how far Arthur and the others support Merlin despite invoking war, proving how true a friend Arthur really is to Merlin.
Also, on top of Merlin's Keep for screens, this site is also excellent, providing literally hundreds (493 for this one!!) of images per episode:
I used it for a couple of the screens below.
Edited 4 total times.