Pressing both mouse buttons on boot brings up the Early Startup Control menu. From here, you can change boot options and display options, and get a diagnostic on any trapdoor slot boards plugged into the A1200.
The Display Options allow you to start your Amiga in alternative modes from PAL/'Best Available' (AGA), the default settings. Sometimes selecting Original or Enhanced helps old games to display properly. Just select the option you want, click Use, then Boot.