How do walls work?

At first i thought walls were a fantastic way to setup defence lines, where enemies would face a real tough time in breaking through, since you could have 3 infantry stacks linked together, each of them getting +1 defence and the attacker would have to fight 2 stacks combined at once.

Except it seems that its used for nothing but sending sucidal units to magically stop the enemy for 1 turn. How 2 militias managed to stop 100+ bombers is a complete mystery to me.

I setup a defence line right outside a city with 20+ tanks in it...i had 30+ infantry total, about a dozen bombers and almost a dozen militias. These were divided into 2 stacks which were linked together.

He attacked the defence line with his tanks and won...which would have been completely impossible if the 2 stacks had been fighting together. Infact i did almost no damage to him while i completely lost one stack and part of the second stack.

Someone please tell me how defence lines work? What determines which actually fight?

Defense lines work, in my opinion, by giving you an extra turn to react to someone's attack. Instead of them being able to easily take your cities, they have to break through your walls first.

What determines which units fight is which side you attack. I don't think there's a way where all the units in the defense line will defend, it's only the two stacks on the side of the attack.

Yes but i had two stacks linked, however not all units in those 2 stacks fought?

Then I don't know what to tell you about that, because those two stacks should've been in the battle.