Fortifications for beginners

I have just started playing here, absolutely love it, but I can't yet understand fortifications. I have tried to search for the info, but no luck.
What is the difference between the various types of fortifications? It seems that unlimited isn't actually without limits... and what does adjacent/chained mean?
Are there any circumstances when you can fortify more than once in the same round, i.e. from A to B, and ALSO from C to D in the same round?
Is there somewhere that explains all this in detail that I have missed?
Thanks very much for any pointers.
What is the difference between the various types of fortifications? It seems that unlimited isn't actually without limits... and what does adjacent/chained mean?
Are there any circumstances when you can fortify more than once in the same round, i.e. from A to B, and ALSO from C to D in the same round?
Is there somewhere that explains all this in detail that I have missed?
Thanks very much for any pointers.