How did the Conquistadors get the Crystal Skull?

First let me start by saying I think Kingdom of the Crystal Skull was great. However I watched it again and there are just some bits that don't make sense. I like to think it is just my understanding of the film and not a gaping plot hole, since my knowledge of Star Wars is at a level where I can satisfactorily explain anything that others may see as a plot hole. As such I believe there must be someone out there who can do this with Indiana Jones.

So, with that introduction, how did Francisco de Orellana and his Conquistadors get the Crystal Skull in the first place? It seems that got as far as inside the pyramid in Akator, since they come across some armoured skeletons. It's even possible that the Ugha natives reset the sand trap entrance to the pyramid after they'd entered. But how would the Conquistadors have opened the door to the chamber of the thirteen skeletons WITHOUT a skull themselves? Then once they had the skull, how did they get back out? There was no flash flood to shoot them out like with Indy, and they presumably could not have got back up the retracting stairs.

The only explanation I can think of is that they entered the same way Indy and friends exited the temple: via the well-like thing. With ropes to climb in and out, they could have by-passed the complicated pyramid main entrance. Once inside, the chamber of the thirteen skeletons was already open, and the door closed after them once they removed a skull. But then it poses the question of what is the point of it all?

Also, are we meant to believe the Ugha warriors reconstruct the masonry they hide behind every time an intruder comes to the city? I guess they wouldn't have to do it all that often, but all the same it seems excessive.

Please keep in mind I'm not trying to be negative, I actually want to know if there are answers out there

 

I assumed the conquistadors simply found one of the aliens and beheaded it- since the body would've been far too tall to carry. There's actually a piece of artwork included on the dvd which supports this theory, and features a headless alien body being carried away by the other aliens. (Or being carried by natives, I'm afraid I don't recall for certain.) I believe the art was created for the Akator set, and may actually be present somewhere in the film.

 

Like RAIDERS OF THE LOST ARK and LAST CRUSADE, CRYSTAL SKULLS had an anti-climatic ending. Only TEMPLE OF DOOM managed to avoid this flaw.

 

It's safe to say that 1 or more conquistadors stumbled apon the skull and they tried to make a run for it.