The game isn't completed. In the .rar file you will find all the original level design planned for a more complete version of the game.

You will also find all the source materials planned to be in the game (such as sound effects). 
Use WASD to move, WASD + SHIFT to sneak around (and make less noise) and WASD + SPACEBAR to run.

You will notice that the IA is kind of buggy.

In the original concept, you can see all the enemy patterns (marked with red lines)
and the tiles that should be noisier (all colored in red)

Where they're there, keys are essential to collect to advance in the next room.

Keys should have been noisy to carry around. When you change room, you lose any key you have with you.

In level 3-2 there should be a "golden door". The "golden key" is found at the end of 4-4.

At that point you should backtrack every room until 3-2 and use the golden key to access area 5.

All the levels between 3-2 and 4-4 are created to be engaging also on the way back.

The same happens between 5-1 and 5-4. In 5-4 you find the golden key but a "boogeyman" should begin to chase you.

At this point you have to go back to 5-1 and finally access your room and reach your bed to save yourself and end the game.