Chep
Emperor
The Arabs took Alexandria and Cyrene in my Cordoba Emperor game. [...] There's also a nice balance between going for growth and tech for the first two UHVs and fending off repeated attacks by Spain, which usually has superior military tech. Mercs are quite crucial, particularly bringing the Tuaregs and Zanji spearmen over from Tanjah.
Maybe it is due to the Emperor level but yeah in my last 2 games (Cordoba&Spain) the Arabs conquered Alexandria and expanded (albeit slowly).
When dealing with Spain I found this usually helped (also is the case for other civs that start at war with you):
wait 2-4 turns and then use Skirmishers/Horse Archers/Berber Cavallry to capture their workers mining the Iron and pasturing the Sheep.
They will make peace with you and not having a single worker should keep them back for a few turns.
this gives you 10-20 turns to prepare for your assault. you still might have to go with Berbers vs armored lancers if you're unlucky but still, I managed to take Pamplona and their other city east of Leon.
Then again you might make peace or just wait for their counterattack, but they should be rather easily dealt with.
Portugal...yeah, their Knights of Avis are quite formidable city defenders but on the offence you don't have to fear them and they usually go down in my games between 1200 and 1250

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Does the AI get some sort of constructing bonus on Emperor? Because it manages to grab quite a few wonders from me, even when I have a headstart in producing them.