If Castle release the code under A BSD license you can do whatever you want with it. You could use huge swathes in your program and never even mention it. BSD licenses only work as commercial entities if there is a large consultancy business ontop (as with say Samba) or you have commercial companies funding it as with many of the Apache foundation projects.
If the license is under GPL you would have to release your code under a GPL license if you used a single line of it. So in practise many companies would prefer to have a commercial license on it. That would be the revenue source, along with support.
That is why many commercial companies in the OS sector release code under a dual license.