I've read about it in walkthrough - it should be above the golden snake with coin slot, but I can't find it.

By "above" do you mean north?
If so, as you look at the small golden snake head west and then north into Kreug's room. The stairs on the south side of the room lead back down to the temple's rear entrance.
The only other exit I know of is via the Naga.

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Rick
Hints, Walk-Throughs and More at The WWHAP

From the walkthrough I used:

"Standing in front of (facing) the snake statue, follow the path left and go along the right wall to reach the front door"

The front door is down a ramp with a lever at the top of the ramp that you pull to "load the outdoors" so you can exit through it.

Coming back in from the outside, the door will be open, you just walk back up the ramp and flip the lever again to load the inside of the temple
: )
Moni