You may have made some decent sandcastles in your time but have a look at The Grande Mosque in Djenne, Mali.  It was first built in 1905 out of mud - because that’s what they had most readily available at the time - and is the world’s largest mud structure. The design is more complicated that I imagined it to be - with sharp wooden defences and a rather graceful (for mud, I think) entrance.  It is now repaired annually in the wet season.  Fantastic.