[Newbie Question] Monster spawns
I'm just into my first game(s) of FE and I have noticed that some monster locations spawn monster armies. I have left a few of these locations untouched near my cities so that I can train my recruit troops by killing these armies off. So I guess my question is two-fold:
1) Is there obvious faults with this strategy that I have missed?
2) Is there random spawns of monsters in the game, making the strategy useless? (I haven't noticed any spawns in the areas that I've cleared totally)