How Do You Get Into the Penguin's Habitat By Using the Front Door?

I have always wondered this, also. I think it is one of the many tunnels in the HQ. It starts in the Penguins HQ, then exits in the lemurs habitat. I know it ends in the lemurs habitat because in the episode "Kingdom Come" When Rico burst the door with Dynamite after Skipper said "Rico, we don't have time to do this pretty". But chances are it is just not a one way tunnel. If intel is correct, I think it might be a conjunction tunnel that leads out all the way through the zoo, if not in the sewer too.

I hope this answers your question. ^^

Maybe it's somewhere in the sewers, or the tunnels they dug in Madagascar the movie...

Yep I think it's connected to the sewer, because Rodger was able to get in their HQ and when Hans took it over the penguins were trying to get back through the sewer

Exacleh. It would be awesome if it exited into a rainbow magic land, but I'm pretty sure it just exits through the (ew) sewer :P And the sewer has many passageways that leads to habitats that have sewer holes in them, like Marlene's!

There is also the fishbowl entrance, and the tunnel thing in the wall that Private's First Prize covers up. I think the tunnel leads to the back of Marlene's habitat, like in the episode Work Order :)

Actually, I think this door that you are showing exits differently in different episodes. Sometimes it leads to Kowalski's lab, or the furnace place, or just a hallway we can see until it turns, vanishing from our sight XP yeah...