I hardly have the heart to do this in much detail - and maybe some fans won't be ready for such analysis - but this is an overview of how our beloved Merlin peeps built Avalon for the final episode. A few different locations were used, and different places within those locations, so it's certainly generated some confusion and discussion. This is how I think it worked, and of course any further ideas will be very welcome.
By the way, the number of different locations used generated a few 'continuity errors', which were noticeable to a Locations Geek even the first time round. When I mention these, I am not at all intending to nit-pick or imply that I expected them to do a 'better' job of it. I think all of this was absolutely marvellous, and can only admire Colin and Bradley all the more for tying the story together emotionally.
Each bit of commentary refers to the screen shot below it. (Screen captures are courtesy of
imaginary_lives. The images, of course, belong to BBC/Shine, and are used with great respect but without permission.)
[1] We first see Avalon, both Isle and lake, as Kilgarrah flies in bearing Merlin and Arthur (below).
The Isle is CGI based on Glastonbury Tor. I don't know where the overall landscape (sky, mountains, hills, shoreline, and lake) would have been filmed, so will mark that as unknown. It might have been filmed especially for Merlin, or it might be stock footage.
[2a] Merlin and Arthur are on the ground on the shore of the lake.
This was filmed at Speech House Lake - not the bit of shoreline that we're used to, which is all bare dirt, but a grassy area further along. The boat that will take Arthur to Avalon waits behind them. There is a little concrete jetty just here, so they probably moored the boat to that.
[3] Kilgarrah talks with Merlin about Arthur's fate.
The larger shot of the same scene obviously wasn't filmed at the same place! The grassy area and the lake are all much larger in scale than Speech House Lake provides for. While this location rings vague bells, I don't know where it is; so unknown. (By the way, note the lack of the boat behind Merlin. Obviously most of us are too caught up in the intensity of the scene to notice such things, or at least not the first time round, so they took the risk and went with it.)
[2b] Merlin and Arthur on the shore of the lake.
Definitely Speech House Lake, and the boat is there waiting. (You can see the trees and their reflections from the opposite shoreline, as Speech House Lake is very narrow. These won't be there once Merlin sends the boat on its way. Again, they're taking the risk with some 'continuity errors'. This underlines the different locations used in putting this scene together. Poor Colin and Bradley must have had to go through all this so many times.)
[4] The reverse angle shot of Merlin and Kilgarrah talking.
Colin and Bradley seem to be filmed against a green screen here, as their edges don't 'blend' with the sky. The sky and Kilgarrah would have been CGI'd in. I'm not sure if this means they filmed Colin and Bradley against a green screen on location, or in the studio.
[5] Merlin lays Arthur down on the grass, and looks across at Avalon.
With the patchy grass, and the shadows of trees so close behind them, I'm assuming this is Speech House Lake, with the camera looking in from the lake or the shoreline.
[6a] Merlin's view of Avalon.
As mentioned above, the Isle is CGI based on Glastonbury Tor. Note the tall slender tower on the Isle. I don't know where the lake or the shoreline is; so unknown. The grass is too wild to be Speech House Lake. I am hoping it's the same shore and lake as [3] above, because otherwise that means yet another location for Colin to have filmed this scene in (he's about to throw Excalibur into the lake).
[7] Merlin ponders Excalibur.
With the trees so close behind, I'm assuming again this is the grassy shore at Speech House Lake, with the camera looking in from the lake or the shoreline - but from a lower angle this time.
[6b] Merlin's view of Avalon.
As above, but with Merlin throwing Excalibur into the lake.
[8] Freya (bless her), our Lady of the Lake, catches Excalibur and takes it under.
This was filmed at Speech House Lake, with a female diver standing in for Freya.
[9] Merlin places Arthur in the boat.
This was definitely filmed at Speech House Lake, and we are now back on the familiar piece of 'bare dirt' shoreline.
[10] Merlin sends the boat off to Avalon.
Again, this was filmed at Speech House Lake, at the familiar location. We track across in this shot to include Merlin (from the back) watching.
[11] Merlin watches the boat float away.
We cut to see Merlin's reaction as he watches Arthur being borne away. (Poor Colin! He's shed so many tears on Merlin's behalf.) This is again the bare dirt bit of shoreline at Speech House Lake. (The trees behind him are different to those in the background him when he threw Excalibur.)
[6c] Merlin's view of Avalon.
When we cut back to watching the boat make its way towards the Isle, we have switched to the CGI vision of Avalon. The boat was filmed on Speech House Lake, and then added to this shot using CGI.
We then cut to a scene in Camelot where Guinevere is acknowledged as Queen.
[12a] A modern-day view of Avalon.
When we return to Avalon, it is clear that much time has passed. The Isle and the mountains behind it have been worn away. The tower is no longer a tall thin spire, but instead looks like the real St Michael's Tower on Glastonbury Tor. I guess we assume that the tower has been partially destroyed over time, or has been destroyed and rebuilt.
[13a] A road near Avalon.
We pan across to find a much older Merlin walking along the road. This was filmed in the road outside Hunstham Court Farm, in the Wye Valley.
[13b] A road near Avalon.
[12b] A modern-day view of Avalon.
Poor Merlin... Still waiting for his beloved friend to return. I am assuming that this was filmed on location against a green screen. The green screen may have been only high enough to block the gate and fields that are here, and so the sky is real (which add to the verisimilitude).
But we know that one day Arthur will rise again. Kilgarrah promised. And in the meantime, he is at peace.