[Norfleet]

Originally posted by Merry Jolkar:
So, you're saying that if I ally with a human player, we can both send troops into our common enemy's province, and our troops will fight the enemy, and not fight each other?

No. However, you or your ally can provide "air support" to the other, if one of you attacks the province on the movement phase: The other can cast spells like Seeking Arrow, Fires from Afar, Ghost Riders, Phantasmal Attack, Flames from the Sky, Murdering Winter, etc., to thin out the opposition prior to the ally's army attacking.

Also, how can I give allies sanctuary

By interposing your territory between his and whatever may be trying to kill him.

and how do I pass through allied territory?

One party marches through the other party's territory, who agrees not to interpret the action as an attack. If fortresses are present, this is simpler, as you can simply march through without attempting to storm anything.

(Actually, a more basic question: how do I form an alliance?)

It is something that is agreed upon by the parties in question: The game does not actually support this in code.

And, yes, can I form an alliance with a computer player?

No.