Thank you for playing and for the nice comment, yes buying spikes, hidden spikes, etc from the shop is meant to kill the knights, there are also shields in the shop which can protect you from a certain number of sword hits from the enemies depending on the level of shield you buy (there is wooden shield < then iron shield < then golden shield < then diamond shield). There are also some small puzzles in the other zones which require you to use tile shapeshifting.