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The ability to merge/time attacks would greatly improve the effectiveness of alliances to work together. Right now, attacks only merge if they arrive at the target close enough together.
My proposal is that if you launch an attack on a city that is already under attack by your alliance, you would get a prompt that says "merge attack ?". If you choose to merge, then the attack is merged. The travel rate for all squads is adjusted so that they will all arrive at the same time.
This feature would allow multiple alliance members to attack a single larger target far more effectively. If they all attack separate, then the larger opponent can usually easily defend 3 separate attacks -- but 1 combined would result in a much more challenging battle.
The amount of merges to an attack, could be a feature of the radio tower at a certain level. So you are only able to merge if you have your tower upgraded to that level.
I seem to remember that Ironworks has already posted that something along these lines is in developement. No idea about the details and how it will work, presumably when it's ready they'll introduce it, tell us and we'll see.