Siege of Constantinople

It's the one in which you're the defender, right?
Assuming all your gens and troops are up-to-date, a three-pronged assault will work.

Southern force to recapture the SW city, Northern force to recapture the NE city and central force to defend against and smash the enemy offensive.
Then, launch a general offensive to wipe out all remaining enemies.

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If you still struggle, prepare for the three-pronged attack, as mentioned. However, rescue whichever garrison still stands and use the other to block the enemy's attack from that direction.

Once the central forces are defeated, march along both lines to retake the two positions and knock out the enemy spawn points. Be sure to march in force because the enemy will keep spamming troops every turn.