An underground world of magma!
It was not as though Riezel had never seen such a place before.
Not only had he seen one, he had seen several similar places.
For example, at the Neutral Zone bordering the Ginas Kingdom and the Rebru Kingdom on the Akasha Continent, there was the Reinayl Ruins in the desert. At its deepest point lay an underground magma region sealing the Fire Dragon King. It was there that Riezel had first clashed with Chalti Shimaraffs, the "Ice Maiden" of the Holy Temple, and struck her into the magma, nearly killing her.
In Labyrinth City Orario of the DanMachi World, the 44th Floor of the Dungeon also had a volcanic region inhabited by groups of Blazing Demon Rocks. Though it was not a magma zone with lava everywhere, its terrain was not much different from one.
On top of that, Riezel had once traveled through the Overlord World as an adventurer and defeated the renowned Lava Ruler in the lava region of the Angelicia Mountain Range. Back then, he had even charged straight into the magma and fought the Lava Ruler within it. All told, Riezel had experienced quite a few magma regions.
Yet even so, Riezel had never seen a city built in magma.
Like a city on water, it stood directly in the magma, surrounded on all sides by red rivers of molten lava. Only at the city gate was there a drawbridge, already lowered to connect with the nearest rock wall, where a staircase had been built flush against the wall.
The stairs spiraled upward along the rock walls of the entire underground space, eventually connecting to the rocky ceiling and disappearing into a cave.
That was perhaps the proper entrance and exit.
Unfortunately, Riezel and the others had not come in