Do you want to put forts at the border or one province in?

[Wizzington]

The forts control the provinces next to them. So the enemy could enter the gap, but could not advance further except to one of the forts. A single fort would indeed be able to control a 3 province border.
Keep in mind one has not to completely siege a fort to move past it, it only has to be under siege.

Incorrect. Once you enter a fort's ZoC you can only go at the fort or go back the way you came, and this is true until you siege it down.

If it looks a bit different in the youtube previews it's because that build isn't the final one.

 

[Wizzington]

So if we go enter to the Zoc via the fort province, we can't go further ?

Correct, though you can double back and enter a different province if you border other provinces in the fort's ZoC.

Basically, whenever inside a hostile's fort ZoC (including standing on the fort) you may only do one of four things:
1) Go to the fort (if not already there)
2) Go back the exact way you came
3) Go to an adjacent seazone if you have transports there
4) Go to an adjacent friendly-controlled province that is not in a hostile ZoC

 

[Wizzington]

So did you guys give any thought to making forts cheaper and only giving them control of their own province? I think this cross-province stuff is a lot less intuitive to many people... we'll see, maybe it will make more sense in-game.

No, but we improved the interface significantly in the final build. Should be a lot clearer.

 

[Wizzington]

So you wouldn't be able to move into a friendly controlled province inside a hostile ZoC (fort on the border)?

Right, though forts can only control provinces with the same owner.

 

[Wizzington]

So my 100k army won't be able to push deeper in the enemies lands even if I leave half of my army to siege the fort? Why is that?

Because then the whole fort system would have absolutely no point.