You have been left behind to defend the city in the off chance of an attack. Defend the city at all costs and put the spoils of war to good use.
Shift -- Sound Space -- Continue Ctrl -- Menu Arrow Key -- Move Mouse -- Aim/Shoot/Detonate
A context menu should be shown when right-clicking or, on touch devices, long-pressing in the game. This menu especially allows to play in full screen and control the volume.