Daarkarrow wrote:

 remain always inside the walls

"Remain inside walls" sounds good for battles with small unit counts, but in fights against big Armys, the main issue is: By moving in to walls, jellow conceded 2/3 of the entire battlefield to blue, so that jellow now can't deploy the mayority his army (simmular in size) because it's outside the battlefiled that blue picked, any every tile is occupied.

So while retreating behind walls is good for small battles, it's is a losing tactic in battles with 50+ units since the defender won't get to deploy his army, becuase the attacker can pick how the battlefield generates, and fill it up with his units before the defender is alowed to deploy. (also there is no guaranie the flag even spawns inside a citys walls

Daarkarrow wrote:

 always waiting for them to attack

This must me sarcastic, in a game where the attacker wins the mayority of all battles in multiplayer.

There is a few reasons for this: 

1. Attacker can kill of the units the defender is alowed to initialy deploy, before the defender is permited to even deploy the rest of his stacks, or even move them in to the battle field. This is especialy a problem when facing high damage compositions like longbows or conquisidors.
2. The Attacker get's to stand on top of rienforcement stacks, so that the defender can't even deploy them (this is ofthen banned in MP)
3. The Attacker get's the mayority of the battlefield space, so that the defender can't even deploy all his units (and even if they can deploy some, they can certanly not move around
4. The Attacker picks the Terrain and layout of the battlefield, this will almost certainly be to the disadvantage of the defender

Daarkarrow wrote:

 move the rest of my armies to help on the situation.

Moving an aditional 50 units next to the fight, is a mute point, since u won't have the space to deploy them, or even enter the battlefield.

So as summary, while the advice works, in small battles with under 20 units (if u play singleplayer, u rarly wil see bigger ones), in big battles the advice backfires hard.

Thats why im asking:
When facing big armys (50+ units), how does the defender need to prepare and fight, to sucesfully fight a big army ?