Septerra core is an old game who's native resolution is something like 800x600 and needs to be run in windowed mode. If you want to run it in fullscreen yet still have it in windowed mode, install compiz (comes preinstalled on ubuntu) and CompizConfig Settings Manger. Go in to the compizconfig settings manager and enable enhanced zoom desktop. you can use the zoom function to zoom to fullscreen then use the 'Toggle zoom area lock' to hold the zoom in place and allow you to interact with the game.