There is no need for a click-through licence. That kind of thing mainly happens with commercial executable software downloads. With source code that can be copied there is no need and you do not see that kind of stuff with GPL software either. The point is that by default, you cannot do anything with someone else's source code. If you want to modify or use it in your projects you have to show that you have a licence to use it. So, there is no need for Castle to keep a record of you accepting the licence. You either do not do anything with the source code, then the licence is immaterial, or you use the source code in one of the ways permitted by the licence and then you have implicitly accepted the licence because without accepting it you would not be allowed to do the things you do.