The Cloud Tombs of Praxius are the flying pyramidal structures on the planet of Praxius IX.
History
Thanos gave Nebula and Gamora a mission to obtain the Orb, inside one of the tombs, Nebula went in without waiting for Gamora and fell into a trap. Ensnared by an impenetrable Laser Thorn Energy Net, Nebula found herself at the mercy of the inhabitants and needed to be rescued by Gamora.
The tombs' line of defense are Ion Missile Turrets, that fire immediatly on the sigh of intruders.