Great to see that someone else is thinking about ancient history of ponies :>
What's more, I once draw a similar pictorial representation of ancient pony history. Maybe I should digitize it and put it somewhere online, too?
This is definitely a STARGATE! :>
This ancient ruin is located somewhere on Earth in the Human World, from very ancient times, where both worlds were once connected. But there was a great war between power-hungry Humans and noble Equines, which caused Ancient Alicorns to tear the spacetime and split these two worlds (I call this event The Great Divide in my personal head canon), for the safety of both races. This spell can be reversed only when Harmony will return to both worlds.
Now, after thousands of years, Human Race discovers these ruins back again.
The hieroglyphs and pictures are an instruction of how to activate the Stargate and open a portal to the Pony World.
See this slanted ramp leading apparently to nowhere? This is the place where the actual portal appears when activated, and the ramp is to facilitate passing through it.
There is a similar temple somewhere in Saddle Arabia, with a similar instruction of how to activate the other end of the portal. It needs to be done from both sides at the same time to work (which implies the Harmony must be present in both Worlds already).