Minimised Stonehearth to check the forums, brought it back up again, and got this message:
Assertion Failed: i != end(c:\radiant\bamboo-remote-agent\buildroot\sh-ob-build\stonehearth\source\horde3d\source\horde3dengine\egRendererBase.h:65)
Game then crashed, taking my painstakingly constructed fort with it 
.