It's in a very high and isolated area. Before roads and trains, it was really difficult to reach.

I think this one happened rapidly because of conquistadors and plague. A rapid empire collapse making the elite location untenable and unaffordable.

I recently read 1491 and it was excellent. It describes how these immense civilizations were in the New World, and how rapidly they collapsed as hogs infected with diseases accidentally escaped the explorer's camps and killed of 90% of the populations.

It talks about De Soto seeing the banks of rivers filled with dozens of cities of thousands of people, and then two or three years later explorers coming by and finding nothing but ruins.

That seems much more of the type of collapse that would just leave entire buildings empty and abandoned.

There's a two day trek and a four day trek. We did the two day which means you walk for one day (6 a.m. to 2:30 p.m.) and then tour the place the next day. The hiking day was quite a bit of climbing, about 600 meter altitude change, but the path was decent and there weren't any scamble up climbs. You visit the village where the engineers who built Machu Picchu lived, a cool site in itself. In mid afternoon, you come over the rise and look down on Machu Picchu. It was superb. Overnight, they (the tours) take you down to a hotel in Aguas Calientes. I don't think I'd do the four day. The last two days are identical to the two-day trek but you stay in tents every night and the bathroom facilities are sketchy. There's also the option of taking the train in and seeing Machu Picchu without the trek.

Not hard at all. You can take a bus, then the road ends and you basically have to walk up a big hill to reach the city

It is at high elevation, but beyond that it's not particularly hard

No. We're actually back home. I don't like posting when I'm on the trip. Home security.