There are several alternatives:
All open documents can be closed at once by pressing the corresponding toolbar button:
Alternatively, you can choose the File | Close All menu
The main menu or simply menu is a horizontal bar with clickable commands anchored to the top of the main window of Atlantis:
command.
Note: you will keep the active document open on its own if you press the Ctrl key at the same time as you use the "close all documents" command.
See also...