A sandcastle was a structure built of sand, typically constructed by beachgoers.
A sandcastle presumably built by the Thirteenth Doctor and friends. (COMIC: Fear of the Future [+]Loading...["Fear of the Future (comic story)"])
The Second Doctor recalled building sandcastles on Ostermede Six (or Seven). (PROSE: The Astronomer's Apprentice [+]Loading...["The Astronomer's Apprentice (short story)"])
The Phantom Piper told Delphi, in a dream, that "peace is a sandcastle" and "war is the tide". (COMIC: The Phantom Piper [+]Loading...["The Phantom Piper (comic story)"])