A little update on the Dawn of Man: another 5-6 hours and I reached the Bronze Age (5th out of 6 eras). The game has become reasonably more complex after entering more advanced eras, as more different resources and buildings come into play and raider attacks become more often, so defensive buildings start making sense. That said, I still find it a bit too easy, at least on 'normal' difficulty on a regular map - once you sort out your farming fields and domesticate animals, food scarcity isn't an issue anymore. Resources are plentiful, hunting game meat is abundant, raider attacks are a nuisance but does no serious damage. The biggest challenge so far is increasing the population, but only because it takes time and you actually lose people due to old age, animal & raider attacks, infections and diseases, etc. (that said, you also get newborns to increase the population as well). I personally enjoy it as it still manages to not be boring and I also like the limited scope of a prehistoric town (as compared to other games where you build huge complex cities), but for someone who wants the game to be more challenging it could be problematic (I haven't tried the 'hardcore' difficulty on harsher maps and haven't done any challenges yet though - it's possible that it makes the game hard and just challenging enough!). All in all, it's around 30 hours of play.
@Bluewolf