Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a very good condensed version of the Arthurian legend, based on Le Morte D'Arthur, by Thomas Malory.
After we see
Arthur pull the sword Excalibur out of a rock which makes him the King, some knights protest that they will not serve a bastard boy, so Arthur takes Excalibur and runs into the woods with Merlin. At night in the woods he is scared by screeching sounds that don't belong there. There are snakes hanging from trees, scarey owls, and weird giant millipedes. He grabs what he thinks is a tree but it's a large snake and he backs away quickly. (I expected him to hack the snake with Excalibur but I was surprised and glad to see that he did not.) As Arthur walks around we also see a boa constrictor dangling from a tree next to him. Merlin grabs the terrified Arthur and tells him there's a dragon out there and that if he saw it in a single glance it would burn him to cinders. He tells him to sleep and dream, to "rest in the arms of the Dragon." From this I assumed that the creepy animals, including the snakes, are meant to be small parts of the dragon; signs of its existence that can be looked at without being burned alive. I'm all for that.
Merlin tells Arthur that he is going away and they will never see each other again. Then he looks into a room in the Camelot castle and sees Arthur's half sister Morgana who is studying to be a Necromancer like Merlin. She's tossing some dead snakes onto a table next to a cutting board where it looks like she is cutting up a rat, but fortunately she's not preparing dinner, she is making magic potions. Just as she starts to cut up a large green lizard, Merlin says "Batwing, snake skin, is this all you've learned Morgana, to deal in potions and petty evil? He tells her he's going away forever (he can see the future) and Morgana begs him to teach her more magic.
Merlin opens a creaky door and takes Morgana downstairs to his secret lair (which is surprisingly close to the castle, and easy to get to.) He calls it The Coils of the Dragon - the place where his power was born, where all things are possible, and where all things meet their opposites. There are snakes lying around all over the place because they are symbols of the dragon as we've already learned. Merlin starts to cast a spell that will entomb Morgana there. The picture has been cutting back and forth between Merlin and Morgana and shots of Arthur sneaking up on his wife Guenevere and her lover Lancelot, and just as Merlin is casting the spell we see Arthur plunge Excalibur into the ground between the naked lovers, who must have been very cold out in the woods like that. Excalibur also stabs Merlin when it enters the ground, which allows Morgana to cast her own spell to imprison him there in a block of crystal. With Merlin out of the picture Morgana goes off to practice some powerful evil magic, including tricking her brother Arthur into impregnating her so she will bear a son who will grow up to kill him and become king. Later we see Merlin imprisoned in crystal, with snakes lying around, but he's able to communicate with Arthur and the knights in their dreams.
This movie received an Oscar nomination for best cinematography, but much of it is very dark and murky, probably to hide some of the cheaper sets and unconvincing action scenes. This makes it hard to see the snakes in the woods and in the coils of the dragon where they blend in with the rest of the set. Most of the snakes are large pythons but there is at least one Boa Constrictor. The snakes that Morgana is about to chop up are hard to identify but some of them are other types of small pythons. Certainly none of the snakes used in the movie are native to the filming location because it was filmed in Ireland, and we all know how St. Patrick ruined that place.