On the topic of monster spawning:
Apparently increasing the world difficulty level will increase the rate at which lairs spawn new monsters. The values just need to be tweaked so that on normal a monster army spawns every 10 turns. On Challenging it should be 5 turns and on ridiculous it should be every turn with very high health levels. I would prefer the Monster Density slider to increase the spawn rate and defense radius of all lairs. On Challenging like minded tribal level neutrals like wildings and ogres should team up if left alone in far away area. The empty space is what I am mostly concerned about. Act II shouldn't mean an end to Monsters. Some places should be kept remote by blocking the way there with mountains or dragons.
Act I is all about scouting the world and carving out a home for your people. The primary enemy is the world.
Act II is expansion to scouted resources and overcoming the dangers of the wild. The primary enemy is the world. A secondary enemy might be first contact with new factions.
Act III is the exploitation and utilization of those resources to win the game. The primary enemy is other factions. A secondary enemy might be extremely powerful world elements introduced in the endgame in various ways.
I never seen the game broken into 3 acts. Every game I ever played goes like this:
Act 1: scouting the world, Build city, encounter non-aggressive monsters and meet some Factions. Find some quest but these are usally just minor stuff that rarley gets completed due to Act 2 when all hell breaks loose.
Act 2: usally turn 30 or so. All out war with factions. Monsters start to become more aggressive.
Never have an Act 3. Never encountered any of the World areas (I forget what they are called) except the one where you can't enter due to a bug.
So pretty much the rest of the game is war (which I don't mind I like an aggressive AI just wish the monsters were more aggressive) Too busy to explore except to get more resources.