In case it helps, here are the areas in order of difficulty that I've been able to determine.
1. Plain
2. Plain cave
3. Desert
4. Desert pyramid
5. Forest
6. Volcano
7. Orc fortress/Volcano Cave
8. Frozen fields or whatever it's called
9 Frozen Cave/Goblin nest
10. Wetware Wetland and Wetware Castle
11. Watercool Coast
12. Watercool Sea
I am not sure where the tower and fairy forest fit in but the gargoyle in the tower seems to be guarding the entrance to the city before the final areas so I assume it comes after everything else.
Also, I have personally found it useful to build craft characters; I've got a blacksmith to provide master grade gear for my fighters and a bookbinder to provide spells for my mage. Since the shops only sell normal grade items, you end up missing out on damage and protection unless you have someone to craft better items for your people.
HTH