If you want something to happen as soon as you start a map, or as soon as you load from a save file, there is an entity for that. The logic_auto entity fires an output based on level start conditions.

In this case, I have surrounded the player start with a func_door entity in the shape of a cage. I have set an output in the logic_auto to open the door (lower the cage into the floor) after the map has been loaded for five seconds. The output is as follows:

OnMapSpawn, cage01, Open, 5.00s delay