Artifacts are indestructible, however, everything I've read says that building destroyers will eventually just deconstruct the artifact, leaving an intact but out-of-place door/floodgate/hatch a tile or two away from where it was before.
I have heard that hatches (And bridges) cannot be destructed from ramps below them. Haven't figured out a good way to test this yet.
No they can't, the building destroyer has to be on the same lvl to deconstruct them.
Oddly enough, however, thieves can pick the hatchcover from below.
It's complicated.
First of all, building destroyers can't destroy bridges, either from above or from below. Bridges can deconstruct from temperature, or from closing on/under something big though.
The hatch situation is actually pretty easy to test. All you need is a building destroyer.
This situation:
__h___
_____L
side view, _ is floor, h is hatch cover, L is forgotten beast
is 100% safe from building destroyers, even flying building destroyers.
This situation:
__h____
__<___L
< is stairway
is rumored to be safe, although I've never tested it-- guess I'll do that.
This situation:
__h_ ##
__<#\_L
# is wall, \ is ramp
is entirely different, and I suspect that the hatch can be destroyed by the building destroyer, even though the FB will stand underneath it (on the slope) while destroying it.