This looks like a great exercise Lockenheim!
The snake/dragon is very well done ^^
You could have pushed the contour lines more, It looks like it could be a lot more rounder in some spots especially directly behind the snakes head.
Since you have a 3 point perspective and the third point is above the horizon line everything below the horizon would be really foreshortened:
- As sascharunge said, the end of the snakes "tail" should also be a lot bigger.
- You can see an example of this when you look at the pillar you drew where the tail wraps around, it's bigger at the bottom and gets smaller duo perspective, you can use that as a reference to how the tail should look like.
- If the 3rd perspective point would be below the horizon the complete opposite would be true and the top part would be bigger
If you want to emphasize the scale of the creature (even more than you already have).
Try to have a good size relationship like putting something everybody knows the size of behind the creature.
You could put a castle structure in the background which has doors and windows, those would obviously be human sized and that would make the creature look a lot more bigger.
Since you already have a human in front this isn't really necessary, just an Idee to make it pop even more ;D
All in all this is really good! I see nothing really image breaking in the drawing, perspective looks correct (outside of the things pointed out).
I would love to see it shaded, since the tail is a huge cylinder this would make great practice with some cool light setup!