Piston

Summary
A piston is a device that the Forerunners, or perhaps the machines they left behind, installed within facilities like the Sentinel Wall as "doors" to get from level to level within the multi-tiered contruct. The pistons have sensitive control-strips that open the piston when fired open or switched. The piston then splits in two, and allows the Sentinels and Constructors to move to the lower or to the higher level they desire.