There's a great deal more than I can write already published about this site. So I'll just put up a few photos and suggest you have a look at some of the official websites.
You will have noticed and are quite right about the lintel on top of the doorway, there's no way the Incas would have left it like that.

There are loads of pages about  Sacsayhuamán, many of which disagree on details like the meaning of the name, Speckled falcon, speckled head, satisfied falcon, I think it should be Head of the Puma, but then I don't speak the language. At least most are coming round to the idea that it wasn't built as a fortress as such, but the Spanish mistook the zigzag walls to be defensive, and couldn't believe anything that big wasn't aggressive, it was used, according to at least one chronicler, by the Inca Manco to threaten the Spanish rule of the whole area.
Below are some links to various histories of Manco. there does seem to be a diversity of opinion.

Back to now, scroll down a bit to get the perspective on this stone, our guide is of normal height.

Here's a couple of photos of the Sacred Valley, and the terracing that the Incas put around it.
It makes our stuff look a bit temporary, and our water management somewhat half hearted.
Looking down on Cusco from Sacsayhuamán, you can't see it but there's a commercial aircraft, landing in the middle of that, the green stripe is an airfield.