How to ERRODE or DIG a square terrain in SURFACE sculpting mode?

I desperately need to excavate terrain in a SHARP square and can’t find a way. It takes hours of impossible eroding to NO AVAIL in Twinmotion an Architecture oriented program
WHY on earth does a smooth circle show a square but erodes in a circle
can anyone add that very basic feature? square to rectangle manipulable lines to go straight down
as to stop terrain from interfering with sub terrain
THANKS
When I am in the TWIN MOTION forum why do I still have to select an application? Also unnecessary clicking


parts of a building