The doors are only [i]always[/i] open when Iron Banner is there. If it's not active, they're closed, but you can open them.
How, I'm not sure. Every time I see it open, it's just been open. I think it has something to do with using a special item to ignite fires? Google it.
English
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It appears you can open the doors by completing the 'Wrath of the Machine' raid by obtaining an Isenfyre Token; which is dropped at the end of the raid. The Isenfyre Token shoots you on top of the tower where there is SIVA Cluster.