Mkay, I'll answer these.
Defensive fights, no matter what kind of troop you have, will always have you on the defensive because someone else is initiating a fight with your troops. The attackers always are attackers while the attacked are the defenders.
Defensive troops are attacked first since they're outside your city. Offensive troops aren't that much help in defensive fights, so it's advised to keep them out of harms way.
The second battle you have when being attacked is an offensive strike where your offensive troops would push back if the attacking force isn't repelled by the defenders.
Unassigned troops will always be commanded by the governor when there's an attack. If the first two lines of defense didn't repel them, the governor takes charge and commands the wall units to attack as well.
*I'm not too sure about this one since there aren't that many battles where my city is.
*You get the units that have the most troops, such as A, B, C, D, E, F, G, H and I is left out because it has the least.
*Towers take up 6x more space, but guarantee a kill. Turrets are just like defending units, they deal damage like a regular unit. Keep in mind, one kill means just one troop.
*Marauder armies are plundering ones.
*Alarm just alerts all armies in your city that it's being attacked and pretty much tells your troops to recall if they're outside.
*And there is no gate skill. Whatchu lookin at?
And I'm already working on a guide. Link