Now that you mention it, I have to agree with you in regards to the Castle tilesets. They do not have the scenery which captures the atmosphere I am looking for, such as moss and weathered stone.

I'm thinking a tileset with grey hued stone and overgrown vegetation would be ideal, as well as having a backround showing it to be under the surface (rays of light slipping through cracks of the ruined ceiling above) or having a dark sky suggesting incoming rain. Any ideas as to other tilesets which might possess some of these characteristics?

EDIT: While I have still not found the ideal tileset I have in mind, The Fortress of Forgotten Souls seems not so bad, although I am uncertain if it will suit my needs well enough. What do you all think?