Henri opens the door. Behind it is a single dark and featureless chamber with a large and unusual-looking metal circle embedded into the floor. It seems... not quite decorated as the ornamentation seems more like schematic or algorithm... at least it seems to be annotated to some extent, in Azlanti.

Spoiler: If you spend some time to read it
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The thing seems to be a test rig for the constructs created or serviced here. The text briefly describes, without explaining, how to use the thing to achieve this and that, but mostly diagnostics. The thing you can immediately make sense of is "detect magic" mode, where everything placed inside the circle shines, to everyone, as if it was under effect of the spell. There are more modes of operation, but you'll have to spend even more time here, fiddling with some actual construct to really understand how they work.