One of the strongest forms in architecture and engineering is the arch. It's been done, in various ways, since the Roman Empire.
The idea is that all the operative forces push against each other--side to side shear on each side pushes inward toward the middle of the arch, and the downward force of gravity presses down and out. It makes a very strong portal.
Interestingly, it also makes an incredibly strong political positioning strategy and way to divert forces around a central core of what someone really wants to get done. Here's how it works.