They blatantly tried to copy Snakebird as closely as possible. Only the graphics, music, level design, and gameplay mechanics aren't as well designed or implemented as in the original Snakebird. It's free and still enjoyable, but if I was one of the Snakebird developers I'd be annoyed.
I love that there's a British English translation... "Your design sucks" becomes "Your design is naff," "Use your brain" becomes "Use your noggin." Classic. Oh yeah, and 5/5.
To stop the ship from moving off the screen, you can just add an extra condition to the movement ifs, for example: "if (Key.isDown(Key.RIGHT) && _x < 575)" will make it so the ship stops moving right when _x reaches 575 pixels. I used _x < 575 for right, _x > 25 for left, _y > 25 for up and _y < 275 for down and it works pretty well...
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