This is an old stone castle, so its scruffy and uneven and has irregular color patterns.
Its tricky sometimes to keep the colors and values in check, and describe the form accurately. What I mean is, you have values that describe the form, like the light and dark sides of the building. Then you have the changes in color in the stonework, which sometimes fight with the light/dark pattern. You might have dark stone on the light side of the building, for example, which goes against the ideal 'light to dark' way of rendering something.
To add to it all, there are also cool and warm colors of stone, which ideally would be placed to enhance your picture; but since this is real life, those are usually uncooperative as well. Cool colors would be in shadows, whereas the warm tones would be out front. So here we're dealing with a good range of greys, which are all over the place, and I'm doing my best to make them work, and still have it look like the place its supposed to be (which is Enniskillen Castle in Ulster).
Another thing that can help or hinder your drawing experience are the photos you have to work from. Sometimes you get great photos, but more often than not, you don't. Either some nice person who has commissioned your drawing has gone out and taken photos that are: too sunny, too dark, out of focus, while its raining, with cars blocking half the building, etc. etc etc. Or you get photos that have been tarted up with Photoshop filters to look all glowy and warm in the sun, when in fact the building is as grey as old dishwater. And on it goes. So the challenge is to find the truth in there somewhere, and do the best you can to make an accurate rendering. It can be a challenge! Here I'm splitting the difference between all my reference and making the best interpretation I can.