And now

1. The ogre is automatically created when the swamp is created. (There’s a boolean to record whether he’s present/alive. This will be useful in deciding whether the game should end.
2. I can count the enemies currently threatening the ogre.

So:

1. Create swamp and ogre
   
2. Will an enemy arrive?
   
3. Yes – here it is!
   
4. Move the denizens:
   
5. How many enemies currently threaten the ogre?
   
6. Will another enemy arrive?
   
7. Yes – here it is!
   
8. Move the denizens:
   
9. How many enemies currently threaten the ogre?
   

Now I can decide whether to have a battle and then work out the outcome.