This is a simple demonstration of how linked_portal_door can be used to subvert the player's expectations.
When making this map, I started to actually think like a game dev. The player sees something they want (the Portal Gun), and somewhere to use it (the portal-able surface). The change in gravity comes as a surprise, and leaves them trapped and confused. Originally, the hallway and the vertical shaft were the same generic texture, and the "objective" was a cake. I changed the former to make it show up better on video, and the latter because cake is overplayed, and associated with obvious traps (read: I couldn't find the cake model).
The end result is obviously not a level on its own, but could be seamlessly placed into one with little trouble.
