You have full rights over any new castles you create. The only restriction is on symbols - think of these as fonts. You can publish a book in Times New Roman, but you can´t copy the font around, or publish a book of fonts. For most purposes (eg publishing a bitmap in a book or website) this is all the rights you´ll ever need. We don´t require a credit or acknowledgement.
The original maps in SMC are all copyright and may not be reproduced commercially, although we are open to suggestion. For example, if you adapted a single map and what to publish it as a flat file, we´d probably say yes if you emailed us.