Raumschach (1907)

Doctor Ferdinand Maack refined the design of Lionel Kieseritzky's original Kubikschach, which translates from the German as 'Cube Chess' and is the earliest known form of 3D Chess, in 1907 and created Raumschach which is German for 'Space chess'.
A Raumschach board is smaller than that of Kubikschach, being only five 5x5 boards stack above each other and incorporated two additional unicorn pieces as well as two extra pawns into each side.
It was very popular in Germany throughout the early 20th century and widely played.