I think you just need a lot more stone than people think. Ancient structures still standing had massive construction. 3-4 foot thick walls. If I recall correctl

I think you just need a lot more stone than people think. Ancient structures still standing had massive construction. 3-4 foot thick walls. If I recall correctly, the Roman coliseum has arches that are more like 6-8 feet thick. But most structures standing for that long have also been maintained to some degree. Unmaintained millennium-old structures are generally referred to as ruins.