Alternate world - turnstile in swamp and "flying carpet" snake

[elgatocurioso]

I'm looking for a book that I would have read about 1990.  I'm not sure how old the book was at the time.  It is an alternate reality book like the Chronicles of Narnia.  As I recall on a dry spot in a swamp the children found a turnstile standing alone.  Walking through the turnstile tool them to the alternate world.  Anything metal they were carrying heated up to burning hot when they went through the turnstile.

In the world there was a king/wizard/sorceror who had taken over and reduced opponents to ash.  There was a flying snake that I recall was flat like a flying carpet so the children could travel that way.

Thanks for your help!