Press the home button two times quickly. You'll see small previews of your recently used apps. Swipe left on your device until you see Kitty City, and then swipe your finger up on Kitty City to close it. Press the home button again to remove the preview view.
Go to your settings and select the "App" or "App manager" option. At the top of your screen, select the "running" view and then scroll until you find Kitty City. Select Kitty City and then select "Force stop".