The number of monsters in the wave depends on how well you reflect the waves before.
If, during the waves of monsters or between them your dwarves died, the number of monsters in the wave will be less.
If you will be well to reflect waves of monsters, the number of monsters in the next wave will increase.
That's whack... Why not rather just tech level? That would be gloriously effective and somewhat in line with normal RPGs. Anybody here ever play Hellgate London? They also made an attempt to scale game difficulty to player performance and that game is still legendary for how much that mechanic was loathed.
So if I keep 2 dwarfs unequipped and unskilled at all times and sacrifice them in between waves to the lesser waves then the game wave difficulty will stay very simple. That's really the only choice one has. I cannot say I like it much at all. Not very RPG when you have only one real choice (a rather poor one)
I did not even build pits and crap like some players exploiting game mechanics. In my last game all I had is some wood cages. Some wood spikes and traps. My dwarfs are all equipped as well as possible at my tech level + 1x mithril helm and skilled up as far as possible. And I got to lvl11 with minimal casualty apart from several unfortunate incidents where a dwarf or two froze while battling frostlings.
And then boom. Can of whup-ass for my efforts. Wham bam, thank you mam, you are now the proud owner of a new one. Though shalt not excel! No time to adjust to increased difficulty. No indication that difficulty wave to expext. Just one wave = manageable and the next totally, moronically difficult. No clue why until somebody in the know tells you that if you play well enough you will be effed up but good.
And on that note. How much extra damage do dwarfs do when they have for example a lvl30 archer skill with a long bow? How much HP do monsters have? How much armor/magic resistance do monsters have?
A little RPG feedback? Maybe?... Or is it all hidden to obscure the obscure balancing mechanisms?
Look I know its not my game. But dang!