Could it be Franz A Mehlem? A little research showed he had a factory in Bonn on the Rhine 1836 and produced earthenware and porcelain until 1920, so the dates seem to match your item. Items were known as Royal Bonn. Could the castle actually be a crown, since this appears to have been the mark used for his work. Try 'googling' Franz Melhem. There are a lot of sites selling his ware, but several show pottery marks and give more information which you may be able to use to identify this as one of his pieces. There are some marks here-
http://www.porcelainmarksandmore.com/northrhin e/poppelsdorf_2/00.php. Scroll down the page. Though not dated they may help somewhat. You could, of course take it to a local auction house and I am sure they will help with identifying the piece. Sorry I cannot be more helpful.