That remind me of another portal to the depths..
You know, the horizontal or slanted giant stone door that opened to the dark depths where Cthulhu crawled out.
Once I've got a few thousand clowns and FB's roaming down there it'll be just like home 
Also fliers will absolutely ignore the rampway and just fly out and kill everyone. Roof is advised. Marksdwarves are advised.
Instead of a roof you will need a series of bridges that you can open like a giant door. Then all you need is a forgotten beast.
The forgotten beast Cthu.. Snodub has come! A great two-legged squid. It has a thin wings of stretched skin and it has a bloated body. Its green skin is waxy. Beware its poisonous vapors.
It was Rodriguez the Portuguese who climbed up the foot
of the monolith and shouted of what he had found. The rest
followed him, and looked curiously at the immense carved
door with the now familiar squid-dragon bas-relief. It was,
Johansen said, like a great barn-door; and they all felt that
it was a door because of the ornate lintel, threshold, and
jambs around it, though they could not decide whether it
lay flat like a trap-door or slantwise like an outside cellar-
door. As Wilcox would have said, the geometry of the place
was all wrong. One could not be sure that the sea and the
ground were horizontal, hence the relative position of
everything else seemed fantasmally variable.
Briden pushed at the stone in several places without
result. Then Donovan felt over it delicately around the
edge, pressing each point separately as he went. He climbed
interminably along the grotesque stone moulding - that is,
one would call it climbing if the thing was not after all
horizontal - and the men wondered how any door in the
universe could be so vast. Then, very softly and slowly, the
acre-great panel began to give inward at the top; and they
saw that it was balanced.
...
The aperture was black with a darkness almost material.
That tenebrousness was indeed a positive quality; for it
obscured such parts of the inner walls as ought to have been
revealed, and actually burst forth like smoke from its
aeon-long imprisonment, visibly darkening the sun as it
slunk away into the shrunken and gibbous sky on flapping
membranous wings. The odour arising from the newly
opened depths was intolerable, and at length the quick-
eared Hawkins thought he heard a nasty, slopping sound
down there. Everyone listened, and everyone was listening
still when It lumbered slobberingly into sight and gropingly
squeezed Its gelatinous green immensity through the black
doorway into the tainted outside air of that poison city of
madness.
Sound anything like your giant hole? Just replace the names with Urists.
By the way, it should be public domain in most parts of the world so don't hate me for making such a long quote.