I didn't even try to go back after the final boss guy yesterday. Instead, I backtracked and tried to improve my lot in life.
I farmed up a lot of cash, some dark crystals, and a pile of gems. I upgraded all my minions to level 3, and had enough resources to upgrade my browns and greens all the way to level 4.
I have to say I'm pretty disappointed that the Steam version of Overlord II doesn't have achievements. I like achievements. They're fun. And they obviously exist for Overlord II - the Xbox version has them.
I mention achievements, because at this point I should have earned some of them. I certainly should have "Lady's Man" and "Gnome Grinder" if nothing else...
Anyway, I farmed up a lot of gold and stuff. I tracked down a health bonus and another mana bonus. I'm having a hard time finding any good FAQs or guides or walkthroughs... I'd really like some idea just how many upgrades there are for mana/health/minions/spells. I don't even know what I'm missing.