1st thought: you attacked city, not the army inside it, hence you don't get a fighting icon above the defenders. if this also means a large part of your damage goes to city improvements, I am not 100% sure of, but I think it is so. If there are a lot of buildings to be destroyed, that works a sponge for the damage.
2nd thought: fort level 3,5 takes a lot of damage away from the troops
3rd thought: out of 31 units of your army, you have 11 AT, 1 AA and 6 militia. So , 18/31 of your army is not really made up of attack units, have low HP and will die quick (and I do hope the arty was not participating in the hand2hand fight?)
Though you should win the fight ultimately, methinks, you would do so only with extremely high losses, due to the build up of your force vs a heavily fortified defender and likely much damage going to the city.
