You finally arrived at floor 15, and you took a little rest after dealing with those vampire bats and gnolls up there. After you're done, you reach the closed entrance to the dwarven metropolis. The gate is currently glowing with electricity, and the giant metal box in the middle seems to hum as it tries to prevent you from touching it. You turned around, thinking of a way to open this gate, and only then did this tank-sized mining machine appear out of nowhere.

"UNAUTHORIZED PERSONEL DETECTED!"

It seems to be connected with the gate. Perhaps you can negotiate with it? Or you can put up a fight and destroy it. Either way, it won't be easy…