I once stayed in a youth hostel in Koblentz, Germany for $2 that was a centuries-old castle. That gave me bragging rights for a while for the oldest place I’ve spent the night.
If you like that kind of funky stuff, hop a plane for Beijing, take a bus for about 90 miles from Dongzhimen station to Miyun, then another to Gobeikou and depart at the Jinshanling Gate. Here, you’ll see a majestic part of the Great Wall of China, built in the 1550s, where you can get a tour frm Sun Hailong, a Chinese guide of Mongol ancestry, then actually stay the night in one of the 450-year-old brick-and-stone guard towers for $24 US a night, including dinner and breakfast. That ought to give you bragging rights for a few decades. Read the story.