Units can be selected en-mass by left clicking the mouse, dragging the mouse in desired box shape, and letting go. All units within these bounds are selected.
Tip: You can cycle through the selected unit type by hitting the Tab key.
You can also select all units of a given type by double left-clicking on that unit's circle. For examle, to select all mages visible to your screen, double click on a single mage unit.
You can also select all units of a given type by double left-clicking on that unit's circle. For examle, to select all mages visible to your screen, double click on a single mage unit.