Use the arrow keys to move, and CTRL to shoot. S and L represent your shields and your lives respectively. Collect the required number of spars on each level to move to the next. Note, the monsters can pick up the spars as well, so you will have to fight for them.
This was my favourite game on the Commodore 64 growing up. I started writing this remake in 2009, and I have only just managed to upload it today (17 October 2014). I wish I had more time to rewrite my favourite games!
This article here is brilliantly written and features an interview with the original creator.