My opinion is that it is fine as is. I have some castle pix but I'll spread them out over time as they are often the same subject. Another criteria I use to decide which thread to post was the time of last posting of the thread. If a thread hasn't had posts in a while, I'll post something over another thread on a different subject with more recent posts. I'm easily amused in how a thread with no activity for some time gets many posts after I "revive" it. So one reason I haven't posted in the castle thread is that it has had recent posts, most recently yesterday!
I, for one, do consider "castle" to mean fortifications (such as a barbican which may be part of a walled city or castle), a portion of a walled city, a fortress, the Great Wall of China, or the US Embassy in almost any country (these embassies aren't very photogenic) in the context of FoP (or the old PotN).