To enter Alien Mode first enter Kitty Towers by clicking on them on the map.

Inside your office there are 5 clickable zones on the screen, clicking on which plays different sounds.



Each zone is marked with a number on a screenshot. Click the zones in the following order: 4 5 3 1 2
When you're done, click the UFO slowly flying behind the window.



Now you're in the Alien Mode and all your residents look like aliens. You also get an achievement for this. To exit Alien Mode, simply restart the game.

With the available notes, you can play the 12 o'clock Big Ben chime by clicking the zones in the following order:

1 5 4 2
3 4 5 3
5 3 4 2
2 4 5 3