I feel like I must be thick or something's wrong on my machine, but I couldn't damage the enemies in the first room. I pushed all the buttons on the pause screen but swinging my sword didn't seem to have an effect.
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This game involves some puzzle elements. Certain enemies can only be defeated by the appropriate tool. If you look at the bottom of the room you will see an open door. Go through there and get the tool you need.
* Based on what you said I am assuming you were in the water dungeon, through the left side door.