Note: this was written before Orangy Tang's answer
Thanks for the answers! But I don't find them very useful, unfortunately.
I understand you saying me "start by doing small stuff", but I already did that. My English is not the best and I should have chosen other words than "start coding games". Indeed my experience in game programming is small as I'm starting but it's not zero as well. I'm coding my game for six months now (purely casually, hence the long process), and I've been writing simulation renderers for another 5 years (in AWT).
I understand the factor "limited time" to develop it and I thank you for the indication of Notch's philosophy about accessors: I read his blog and found out what he meant.
Regarding the engine, I'll stick with AWT for the moment, not that Slick (which I also tested) is bad: I just want to start with what I know best.
In that context, I hope my questions make more sense and may actually find answers about how Mojang designed their Catacomb Snatch.