I think a lot of it boils down to the fact that whoever wrote or told the prophecy about the cities of gold, knew exactly when/where and who would find the cities of gold.
So I guess they made it purposely difficult to find the cities of gold, because someone or something, knew the future.
Wasn't the mountain the "final" reference point on the map (along with the three cities) - and the intersection point was the location of the "actual" city of Gold?
Indeed it was, yea, but there was no way for them to find the cities of gold from that point on I mean the ancient manuscripts, which were placed in the three ruins, were behind elaborate traps/mechanisms, but there was no such thing in the 4th location, which to me never made any sense You have 3 old ruins, then this super advanced structure in a mountain, with (I assume) no manuscript If it weren't for that dying Incan (sp) telling them about the intersection of lines, they never would've found it Plus I have to wonder how that guy knew what the inside of the cities of gold looked like, given that it was sealed behind "unbreakable" doors.
That's quite right, Zia's father might conceivably have worked out the location but could not possible know what the inside of the City looked like. It's worth mentioning that Mendoza almost came to the same realisation himself - he surmised that as the first three cities and the mountain were within a few days walk of each other, then the final city had to be close as well.