There are more than 1 backdoors there.
1. Door to get in Gummi Ship (in KH1)
2. Door to Synthesize Shop (KH1)
3. Next to Hotel in the 2nd District
Traverse Town is a world in Kingdom Hearts I. Sora came to Traverse Town after the Heartless conquered Destiny Islands. Traverse Town is a world that people go to when their worlds are engulfed in darkness.
Traverse Town is a world people've lost their homes come to.
Traverse Town is in KH1 but not in KH2. Whereas, Twilight Town is in KH2, not KH1.
Clear Traverse Town
I'm going to assume you mean where is Traverse Town's keyhole. When you return to Traverse Town and talk to Leon (after sealing your first keyhole), he will tell you about The Legend of the Clocktower. Go to the First District and enter the Room with all the mechanisms (I apologize, I can't remember the name of the area). Leave from the backdoor, and to your right you should find a ladder. Climb up the ladder and defeat the heartless at the top. Then use the red trinity and bash into the wooden thing blocking the bell. Ring the bell three times, go to where the keyhole appears, and defeat the boss. After that, you should be done.
It is the first world you go to after the events in Traverse Town. When in the Gummi Ship screen go to the world that is north east of Traverse Town. It should be Wonderland.
Return to Traverse Town. You will need to occasionally revisit Cid so that he can improve your Gummi Ship.
Lock Traverse Town.
There are few features a traverse offers that a town and country doesn't. But all in all these two are about the same in things they offer to the driver.
The White Trinity is in the Secret Waterway.
No, but it is seen in cutscenes.
Sora ends up in Traverse Town after his home-world, "Destiny Islands" is overtaken and (temporarily) destroyed by darkness. "Traverse Town" is a world that holds former residents of several worlds that have been destroyed by darkness, so it's almost natural that Sora ends up there.