So, the areas that are divided by these walls are sort of time barrier, and each area belongs to different age.

So Hyne picked up each area (with their own time) and made the WOTC, and the result was the present world map that we have now?
Exactly.

what windmills are we talking about here?
English is not my first language.
I am referring to these helixes who rotates with wind force.

The field is the wind field.

Sounds like one of Einstein theories if you ask me. I mean to have enough speed to archive the time itself.

When you mentioned about the time travelling vehicles , could be the Ragnarock or the Lunatic Pandora?
No,I am talking about wind here,not time travelling.
These winds are created to prevent common vehicles to touch the walls.
It prevents people to discover the walls.
TT vehicles don´t need this because they "pass through" the walls.

The TT vehicles and propolents I know of in the game are:

Lunatic Pandora,Balamb Garden,the chocobos,Garden vessels,Galbadian missiles,Lunar Gate rockets,Ragnarok,Advanced Galbadian trains,Ultimecia´s castle,Galbadia Garden and maybe the White Seed ship.
There would probably be more.Those who pass through the walls are certainly TT vehicles.
However I believe Galbadia´s common trains,cars and the vessels on the crystal pillar excavation site to be common.