Linked Portal Door Concept: Endless Ramp (Source SDK)

This is a very simple concept that was very hard to get right. I had to spend nearly an hour checking and re-checking the heights and widths of portals and walls in order to get everything perfect.

The ramp itself contains only one slope, connected at either end by perpendicular portals, with the end result that you can clearly see yourself when going through the portals. The solution to this would be to build a larger ramp.

One amusing potential application of this would be to set a counter that requires you to go through the portals a set number of times, then disables them, in effect allowing you to have a seemingly endless staircase that is actually only a few steps long.