Clifford D. Simak 
Special Deliverance 

They were six ill-assorted players of the strangest game ever devised. Set on a strange planet, they knew neither its rules or its aim. There was the giant blue cube, the ruined city, the singing tower, the roaring wall of Chaos – all deadly dangerous, all part of the challenge that the unwilling travellers must confront. But what was it all for – and would they survive to find out?