Why has the game got me doing the same challenge I have just finished. I have started it again but I what to know why.
I might as well stop doing challenges.
As mentioned, each time you complete a Cultural Settlement, you gain a new reward for use in your main city.
The first time you complete each settlement, you will receive the basic 'Settlement' building. (ie: Royal Bathhouse Lv1 for completing Egyptians the first time)
Subsequent play throughs will yield either an 'upgrade kit' for the main prize building, which will increase said building's stats & bonuses, OR, an 'Emissary' who can be activated in your Main Hall.
While you will need diamonds to use every single Emissary at once, this is simply a trap - typically you will always have enough 'free' slots to keep the likes of the 'bonus fp' & '+ military unit/s' emissaries activated.
While the Settlements are extremely grindy in nature, only the Egyptian Settlement currently has a stupidly powerful 'time reward' that you'll drool over, and luckily, it is bar far the easiest to acquire! (hint: build, then delete the required Training Camp, and replace it with 2x Nubian Archer camps - these guys can win every 'easy' + 'medium' level fight on their own.)
To give you an idea of just how insanely powerful these buildings are, consider this:
a) Royal Bathhouse Lv6 = 4x3 building, gives 10 random units per day + 12fps
b) Alcatraz = 10x7 great building, requires Lv12 to give 10 units per day.
(NB: will require a total of 5814 fps to reach Lv12!
)
Now, while it's true that you can tailor exactly what units 'Traz will generate, (as it is based on what military barracks are placed in your city), vs. the purely random generation of RB's units, the fact is that RB is absolutely tiny in comparison, and thus, is much more efficient, especially for an early game city where space is extremely limited.