Hi, very nice job, well i got the 3 keys and the gate didn't open , but could manage to hit the minotaur from a distance.
About the graphics, you should keep them simple, remember you are no expert in the graphic part, so the simple the better, a serious character/environment is more complex to realize, so keep that in mind.
To open the gate you need to go near the columns (for now of course!, i know it's not clear!)
Regarding the graphics, yes, more simple the graphics is more problems i will avoid!