Re: A.I+difficulty and naval invasions(vanilla 1.0)
Sodbuster is correct. The Sicilians arrive on a punctual and early scehdule. While playing France on VH they bring a 1/2 stach to Marseille. You can ceasefire and get cash out of the deal, but the stack will remain. Eventually you need to deal with the invasion even though the invasion is rather static. Kill it and it returns. with better units. Maybe this is the only way AI can demob obsolete units?
Back to Spain. The Moors seem to take units from the two north african cities. Lacking patience, this mean two four stacks that will combine if able. They do something similar with France at Toulouse whenever they are not just wandering with a small twit stack on recon.
The English (and sometimes the Scotish) will also send a 1/2 stack to the Leon area. Usually includes mainly peasants and militia.
So, the short answer is that AI cannot put a major invasion of elite units onto your soil for a prolonged campaign. The twit attacks are distracting and do draw resources. I guess in game terms, they do serve a purpose.