Monsters and Their Uses
One thing I like to do is set strength based on elevation. I know that seems odd, but it does work for me. If you have a monster invading high in the mountains, with little chance of the monster actually getting to your main population base, I would set it to aggressive. That is not where I end it though. Monsters default to attack food. I make sure that a monster stuck in the hills is set to attack citizens and maybe even military. This will get the monster roaming good distances, but it will always return to its point of origin. A monster that is placed on a hill near a resource would be set to active. Then of coarse I would set it to attack the resource. And finally, a monster that is placed on flat open land is set to passive, and will attack food, mainly meadow monsters.
With those settings in place, you will find that your monster roamings are pretty good and also challenging, but not overly torturous.
Sea monsters have a little bit different feel to them when setting up. Most times, I will try to place their start points in area at least 15 tiles away from good docking points. This allows players to build docks and piers, without the threat being uncontrolled. The further away I place the monster from these points, the higher I set its aggressiveness. Again, always remember that it is the player that you are trying to please, and not yourself.
Once all of this is in place, you must decide how the monster shall be handled. Will you allow a hero's hall; will you allow the player to build a sanctuary? Many aspects of the game depend on how you first present the challenge to the player. A monster guarding a valuable but not needed resource may be placed and allowed to roam without offering the player a hero. If the resource is not needed, let the player choose how to deal with the problem. But if you do allow the player access to the hero, you must make sure that the player has a chance to summon the hero. I made this error in one of my earliest adventures, and realize now how important it is to allow the player to understand what is going on. If a monster invades and no hero's hall is available, the player knows they must deal with it. However, if a monster invades and the player sees the hero's hall, they expect to be able to meet all the requirements of the hero. That is a true must for good adventures.
With the above in mind, it is important to note that some hero's require lavish items in order to be summoned. When doing this, you must also realize that your adventure will move forward at a fast pace in order to meet those needs. I like to start adventures off slowly, with limited housing, few aesthetics items, and little educational facilities. Because of this method, I seldom allow a monster invasion early on in my adventures. On the other hand, a short adventure with only a few episodes may have all the needed items from the first page of the artist's creation. That is all up to you.
Finally, lets think about the independent monster. When selecting monsters, you are allowed one independent monster in your choices. This monster may be selected from the entire list, other than the two monsters used by the first two angry Gods. I like to use this monster in a much different manner than a lot of players. Artemis for e.g. is the God that holds the Caledonian boar close to her heart. I have planned scenarios where the boar is actually an independent monster, yet Artemis resides in the parent city. The boar was not fed on time? Lots of fun can be had with the independent, and it may invade in the same manner as the others.
I hope that this little advice helps you in your quest for designing a great adventure. I know I learn everyday, and the feedback I receive from players is priceless when trying to improve.
Have fun all