You can reach the watchtower by finding the key in "Moving Bridges" and then beating the level "Going Down"
(How to obtain key: At the end of the first room, break the blocks on the last rising platform to find a door that leads to a "break the targets" stage. The key is on a ledge to the left. )

How to reach the watch tower :,)

The password is left of the vacation island, there's a singular stone block underwater. Break the rock and drop down for a secret area.

World 7 is made up of the red levels on the map, the green levels are world 6

oh I may got it...

As I see, it seems that I beat every stage on World 7(green painted,right?), however, I dont get extra bill for beat every stage on W7.
I played whole stages again and aint find a key except of beeutiful stage.
What am I missing....?

You can only access it once you've defeated Billi' Bones.
Hint: It's hidden somewhere on the vacation you unlock after beating the game...

I don't see anything wrong with the map but I just have question, where am I supposed to find the password? I haven't been able to find anything about it online or in game :(