According to Thea von Harbou's novel, Rotwangs House was originally built by a magician. This may explain the pentagram on the front door and in the room where the robot is seated.
Description of the house in the English novel
THERE WAS A HOUSE in the great Metropolis which was older than the town. Many said that it was older, even, than the cathedral, and, before the Archangel Michael raised his voice as advocate in the conflict for God, the house stood there in its evil gloom, defying the cathedral from out its dull eyes.
It had lived through the time of smoke and soot. Every year which passed over the city seemed to creep, when dying, into this house, so that, at last it was a cemetery, a coffin, filled with dead tens of years.
Set into the black wood of the door stood, copper-red, mysterious, the seal of Solomon, the pentagram.
It was said that a magician, who came from the East (and in the track of whom the plague wandered) had built the house in seven nights. But the masons and carpenters of the town did not know who had mortared the bricks, nor who had erected the roof. No foreman's speech and no ribboned nosegay had hallowed the Builder's Feast after the pious custom. The chronicles of the town held no record of when the magician died nor of how he died. One day it occurred to the citizens as odd that the red shoes of the magician had so long shunned the abominable plaster of the town. Entrance was forced into the house and not a living soul was found inside. But the rooms, which received, neither by day nor by night, a ray from the great lights of the sky, seemed to be waiting for their master, sunken in sleep. Parchments and folios lay about, open, under a covering of dust, like silver-grey velvet.