Simple connector to build castles from 20mm PVC pipes. There are plenty of designs available on this site and this one is optimised for simplicity & sprint speed (vs aesthetics or support of different angles or...)

Depending on the country you live in, this design should support a standardised insulation pipe labelled EN20. For example, in Germany, any hardware store might carry this for about one Euro for two meters (e.g. https://www.obi.de/leerrohre/isolierrohr-starr-en-20-laenge-2-m/p/7550171#/)