My First Castle/Fort WIP

I like the general layout, and how beefy the corners are! I do think the corners could use some texturing or architectural details like you've done with the walls. They are a little "big gray wall" syndrome-ish right now.

Looking forward to seeing how thus develops!

The first thing I'd look at from the standpoint of defensibility is where you'd have archers or crossbowmen posted to defend the walls. These could be as simple as 1x2 technic bricks with an axle hole, or they could be more elaborate brick-built slits. Or you could add some windows. Depending on the technique, you could have slits between the buttresses along the wall, or you could have them on the corners. That would start to break up the big gray walls already. Then you could consider mixing in the occasional plate, 1x1 round plate, or studs-out tile to add some texture and/or some pieces in dark gray or dark tan to add some color. Also consider that wherever windows and doors were outlined, masons would typically use a different type of stone than the rest of the wall, often resulting in a difference of texture and color.

You might also consider having the walls or towers extend out a bit so that defenders above would be out over potential attackers. That would allow them to drop rocks, boiling water, boiling oil, and other unpleasantness on the enemies below just using the force of gravity. If you do take that approach, you could start thinking about where you might want to incorporate inverted slopes, beams, or braces to hold the upper sections of bulging murder holes or hoardings.

Other things you could consider would be some ivy climbing the walls, some battle damage or wear (such as holes, chips, or stonework done in a different color that looks like a patch job), and/or maybe some parts of the defenses in wood (like hoardings or watch towers).  Those kinds of details also break up the monotony of gray.

Also, I typically don't build all the way to the edge of a baseplate. I like to be able to have some overhang for things like murder holes, ivy, or minifigs hanging off the wall, without preventing my module to be pushed up next to someone else's module in a layout. Even if the margin is just a couple of studs' worth all the way around, that will go a long way to making it possible to align sections of a large build. (Plus some amount of margin lets you show off your skills at landscaping, which I always love.)

Ragni raises an excellent point: Who built something (and how they built it) will dictate the details of the finished work. I was throwing out a variety of things that can work for breaking up space, but any build should have an overarching design and reason for the design choices within it. So it could have started as dwarven workmanship of the highest order, but the dwarves left, humans took over after years of neglect, etc. Or it could have been damaged at some point and only partly repaired. Or it could have been built in stages by a series of rulers with different styles and materials. (Many castles in real-world Earth were done like this.) Etc., etc.