Use of a Field Defence Specialist.

[Kavhan Isbul]

No, it will not be. If you bring in reinforcements in an attempt to lift the siege, it will be considered an attack and king Alphonso will be down to 4 stars. If your castle is assaulted, I think it is not considered a defense, as defense is meant to be field defense, not assault defense, and he gets 4 stars only again.
As pointed out, the best of use of your king's field defense specialty is to invade Cordoba with as big an army as you can muster, which in 99% of all cases will cause the Almohads to retreat (when significantly outnumbered the AI retreats, and them counter attacks with a bigger army), and then then use his maximum stars when they attempt to retake the province.

[Martok]

Hey Tony! Kavhan Isbul has got the right idea. Just invade Cordoba (or whichever province you choose, but Cordoba is ideal for this situation), and then wait for the Caliph to launch a counterattack against you. I've also found that for best results, you should lay siege to the castle instead of directly assaulting it--this gives the Almos a little extra time in which to prepare their counterstroke against you.

[Roark]

$0.02:

If I've mustered a really big, state-of-the-art invasion force, I try to pair up a field defence specialist with an assault specialist (or, at least, have the defence specialist tailing very closely). This way, I can invade a province and leave the defence guru to lay siege to the castle with less troops and less fear of a counter-invasion due to his command bonus in defence. My invasion force can then move on to new territory with maximum possible troops.