Music sucks now....
Perikles is one of my favorite games. Unfortunately the alliance rules are a little muddy. What follows are the rules I use to simplify these rules in a somewhat thematic way to smooth out this process. Honestly the rulebook is so confusing in this section that some of these rules might actually be what was intended anyway.
All rules are played as written except for the following:
-If you want to be a main ally you must ask permission to defend a city that is not yours (like the rules suggest). This however gives full control to the main attacker to decide who else can defend the city. If an allied defender wishes to defend the city they must ask permission to the main defender. This eliminates a weird and frustrating situation where you are defending a city as a main defender and someone you would not prefer to be an ally with gets the go ahead to be an allied defender which means you have to be allies with them against your will.
-The rules suggest that anyone can play into the allied attacker box forcing themselves into an alliance with a main attacker which doesn't make any sense. To eliminate this from occurring a player must ask permission from the main attacker to become an allied defender so that the attacker does not have to join forces with someone they want to attack.
The rules as written seem to lead to instances where people can force themselves into alliances with people to keep them from attacking their cities. For instance Sparta is attacking an Athenian city, and a thebian unit gets placed in the allied attacker box forcing themselves into an alliance with the Spartan. This would void the Spartans ability to attack a thebian city/unit somewhere else which is completely out of the Spartans control. With this variant, that thebian player would have to ask the Spartan player if being an ally is ok, which makes sense gameplay wise as well as thematically.