How to prevent the enemy from sending reinforcements for a battle

[twgamer20197]

Did you ever notice these things?
There is a way to avoid fighting two enemy armies at the same time in a battle.
Case A:
When there is an enemy army inside a city and a another army close to the city. And you want to take the city. You need 2 armies in order that the trick to work ( one of your armies can even be just a single general unit ). The trick is simple:
Use one army ( it could be just a single general unit ) to attack the city but don't start the battle yet. Just leave it in siege mode. Then use your other army to attack the other enemy army which is outside the city. This way the other army which is inside the city won't join the battle and not even the garrison army.

Case B:
You need a spy for that. If it's the same situation as described above, use your spy to sabotage the enemy army which is outside the city. Then go attack the city ( in case your idea is to start the battle immediately and not waiting for any turn ). The army that is otuside won't join the battle. However, I don't remember if your spy needs the special skill that both shortens the enemy movement range and causes some casualties. But if your spy has it, surely the trick will work.

EDIT:
Another way is in case you are a barbarian faction that has the option for choosing to fight night battles:
Choose night battle when attacking a city. And the army that is outside the city won't come to join the battle.

[twgamer20197]

Here you have an example of how a gamer takes out several armies ( that are very close to a city) one at a time. First he locked a full stack enemy army inside a city. Then he used another army to take out the armies that were otuside very close to the city one at a time. That part of the video is from 57:02 to 1:16:02


One more example of blocking enemy reinforcements from the same campaign. Watch it from 1:33:21 to 1:33:58