Hey, great work on the 3-point perspective assignment, it really shows that you put in time and effort to set up an interesting scene.
I have little to say in terms of critique of your perspective, which is consistent and accurate throughout the whole piece. Kudos for that! I think the one thing you could improve as a next step is not your perspective, rendering, or composition, but instead building a wider array of reference pictures when composing a piece.
By this I mean looking up references of castles, cathedrals, gates, and so on, and maybe even researching the history and purpose of architectural elements.
- Square vs. round towers? Typically they're separated by a few centuries if they're on the same building
- Round vs. pointed/gothic arches? Also not from the same time-period, and can be remarkably complicated in terms of stone work.
- Flying buttresses or simple wall buttresses? Why are they there? Are there enough along a wall? too many?
- Bricks or masonry? Are the slabs chiseled out of rock, or combined through brickwork? Are there decorative elements or is it completely barren?
- Scale, design and spacing of wall crenellation, logical position of machicolations?
- etc.
To make up for limited research time when constructing a drawing, references are key! To build anything from imagination is really hard, because you really need to master the subject in order to avoid mistakes when inserting details, in this case medieval defensive works and architecture. Very niche stuff!
Once again, great piece, and looking forward to more of your work 
I hope I'm not a downer or anything, I think you're on the path to greatness and this is the best advice I could give from looking at your piece! Cheers!
