Hello,
To activate the Garrison Script you must Click the ESC key on you Keyboard when you just started your campaign. Then you need to click on the Question Mark Icon (?) shown in the Picture. Then you must click on the Show Me How button, shown in the picture under the advisor. This will then activate the Garrison script, you can then exit out of the advisor and continue your game.
You MUST do this Everytime you LOAD a SAVED GAME. This script will run throughout your game while you play, until you EXIT your campaign game. Then when you Start your campaign Again you need to do this ONCE before you play. Its really easy and once you get the hang of it and REMEMBER to do it, there will be no problems.
You DO NOT want to start a game with the Garrison script and then not use it later, the same is true with starting a game without the script then activating it later in your campaign. You should play ONE or the OTHER, NOT BOTH.
Also Saved games, you should RELOAD a Saved game many Turns back like if you lost a battle and then reload the saved game to play it again, As this MAY cause Issues with the garrison script in your campaign. Of course this ONLY applies if you use the Garrison script.
Over all the Garrison script adds a Garrison ANYTIME the Player gets next to or Sieges a Town of the AI. It is best to be careful when getting close to a city because you can have them spawn a garrison and not siege, which is giving them an army, although a low quality one. This is to reflect the Town defending itself when an invader comes to attack their homes, Since the AI is unable to leave a nice garrison, The garrison script will correct this. This makes Sieges in the game more FUN and more of a Challenge.
Of course you can find ways around the garrison by attacking early etc, but that DEFEATS the point of USING the script in the first place. lol If you want an easier game, play without the script.