How do you win?

You need to get enough vp. If you get the required amount of vp to win, the game ends.

Yup. Get enough Victory Points. The Victory Points necessary are given in the newspaper, top right.

there is a second option. if you are three players left and you dont want to fight each other you could all agree on retirering the map. however all three must click the button, which can be found in the newspaper. when all three have done that, the game ends(might need refreshing the map) and you will still get gold if the map is ranked.

(dont think you can in 4 player map. all the other maps there it should be possible)

You should also everywhere in the game click the small (i)nfo symbols.

And first that one in the left top, next to your flag - there you can read, among other things, how to win.