Don't shoot me... I really don't know how to specify it. It just does not look right. The object itself is a little too simple, as compared to the other icons. I would try to add some shadows, perhaps a few windows, or try to hint at the different castle parts (draw a line to show the outline of the towers or such). Sorry, I am not an artist - all I myself would do is try and change and change again until I found it good. So I cannot give a step by step advice on how to do it... I don't know myself.
But one thing you could change: fit the castles better into the available space. Dun Glorach touches the northern coast, Desportach the river next to it, Ailcarraig the bushes above it. I know that it is difficult to prevent overlaps while using brushes, but I'd say that is something an original cartographer would have done: "There is empty space here on the map, let's fit in a castle."
What you could do: when you have settled for a style for the castle icons... don't make it into a brush, draw each castle seperately. There are only four of them. (And for the style: it needs to be something you think is right... you are the artist, we are only the critics.)
Bog and coastline should be no problem. The problem are other icons, where overlaps make the text difficult to read. If you cannot find a space to fit the text between the icons, consider moving the icons. Or removing some of them.I am at this point not sure what there is to be done about the placement of the text. Maybe I'm just getting to a defensive point with my work, but it FEELS nitpicky, and there literally isn't ANYWHERE I could put the Dochaoiran marker WITHOUT it going over the bog and/or the coastline. The others I might be able to fiddle with.
Or, another thing you might try: get rid of the text completely (at least for the towns). If you cannot at all find a way to make them fit, make them vanish!
Sorry if that feels nitpicky... it is just a viewers opinion on the difference between a great map and a perfect map. But I think that this is indeed a great map, and it deserves to be perfect.