I have a question about ancient ruins.

[mdelaguna]

1. A few reasons

You are right. Recycling of building materials was common. Also, often multiple building phases are present. To add a new layer it helped to flatten things out to use the old structure as platform fill. Later buildings toward the surface are more subject to natural or cultural processes of destruction so often digging finds better preserved earlier foundations. Or people are more interested in unearthing earlier remains. Why do some preserve? Rapid abandonment of an area helps (no later scavenging or re-use), or an edifice becomes enshrined and maintained in social memory and is preserved. Also walls & roofs are often made of more perishable materials (wood, plaster, mortar, and will collapse over time or were affected by fire. Some really well built buildings hold up better. (I’m an archaeologist).