his code style is rotten. i'm no newbie and i can understand his code and i can state because of that his code style is rotten.
even he states his code is rotten (well, not "rotten", but a mess..).
btw, an engine is nothing big. but a big thing to understand how it exactly works. the components of the engine can get quite big, too, depending on what features you want to add..
the most complex thing in programming is not how to solve a certain problem, but how to organize tons of "problem-solutioners" together to make it a working full program/engine..
in graphic-terms: you can make a particle demo, a bumpmapping demo, a stencilshadow demo, an animated mesh demo, a small rigid body demo, a simple fps-demo, a tree-generation-demo, and all. but making it all work together, thats hard. very hard. and not because you don't know how to do it actually. but because you have to do it, and its rather complex to orient yourself then in it.