It lets you get 1 of the most advanced millitary unit from a city at the cost of population. So, If you just got gunpowder, you would draft a musketman. To use it, highlight a city and click the button DRAFT above the slavery and sufferage buttons.
Beware: this costs some of your population and can only be used once per turn per city.
hum
It's the idea, but not exactly so.
I think its 2 pop at (mech) infantry.
no, it's 3 pop for mech inf (It's indeed 2 pop for infantry)
Drafting :
What does it do?
It gives you one unit (the best available) out of this list at the cost of population :
- (I don't know before maces, probably LB? or archers?)
- macemen (1 pop)
- musket (1 pop)
- riflemen (1 pop = best value for the pop)
- infantry (2 pop)
- mech infantry (3 pop)
There is also a unhappiness penalty of 3

each time you draft in a city, for 10 turns. This penalty stacks in value and duration (like slavery).
The draftees have half the xps a normally built unit would have.
Meaning that if you have what it takes in a city for level 3 (5 xp) units, you'll only have level 2 (2 xp) draftees.
The draftee is available as soon as you click on the button = can move in the same turn.
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You need to be under nationhood civic (requires nationalism).
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You can draft up to 3 units in a turn (only one in a given city).
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a city needs to be size 6 at least, and to have 10% of your national culture in it to be draftable.
- You can draft AND whip AND $rush (
although $ rushing and whipping only finishes the current production in the city, so it's 2 units tops in a given city in a given turn) in the same city.
- note that nationhood gives 2 happiness points for a barracks, making the unhappies less painful
- the best units to draft are riflemen, because they cost only 1 pop point, and are strong enough to make a difference. It's even better if your UU is rifleman derived
Is it useful?
Yes it is. It's the fastest way to bring up an army.
You "slow build" or whip the siege units, and draft the bulk of your troops.
After 5 turns, you can have 15 riflemen and a good deal of whipped cannons/catapults/trebuchets.
This should be enough for an early gunpowder invasion.
If you're gunning for the win, you just don't care about unhappiness : push the slider as high as it gets and continue the drafting all the way.
a 10 turns war gives you 30 units. Unstoppable.
What the bad thing about it?
1) you're not running free speech, or vassalage, or bureaucracy
2) you can't really have a mixed stack
3) You have a big population loss.