Arrays, typed arrays, array that can be filled using function, dynamic arrays, arrays and a side order of arrays.
They are vital, the more you know about them the better, if you know everything about them then you'll already know what they can do.
Every enemy, every weapon, every spell should be stored in an array of some sort, depending on how you organise your game. I assume that one thing you'll want to do is upgrade weapons, so you might have a few variables for an array full of weapons, and these are usually best assigned as types. Then when factoring how much damage a weapon does, or how fast the animation runs, or the likelihood of the weapon getting damaged, those arrays come into play, along with player stats for strength and agility. It is possible and actually quite fun to have traditional platformer action, add in a weapon and the ability to upgrade it is almost a no-brainer. Really the only 2 aspects of that you have to really worry about is the speed of the attack, and the damage dealt, your looking to arrive at those 2 figures after some math. I would have a starting set of parameters for each weapon, then by upgrading the material you could make it faster and lighter, stronger, make it deal more damage etc etc.
I think this is pretty prevalent in the Castlevania series, the whip and armour can be upgraded, and with replacement graphics I think it would work really well. With 2D you can go to town really, as a completely unique sword for instance would take a few minutes to create whereas in 3D it would take about a day.
I think you could do worse than playing some classic platfrom adventure games...
GODS (Bitmap Bros, Amiga + ST) - This is a real gem, has a distinct and classy style to it. There are shops to upgrade weapons and buy health etc, it is quite arcadey, but really it's the style that is unique in Gods, a damn pretty game even when limited to 16 colours.
Beast2 (Psygnosis, Amiga + ST) - This one is much more puzzle orientated than Gods, it actually has some 2D physics puzzles and was possibly the first game to have that level of interaction. It has a distinctive style too, but only as distinctive as Psygnosis' own style, which tended to be copperline background, jet black parallax layer, then the platforms over that. What I liked about this game was how tangible things were, items had a much more believable form than with other games.
If you can make a platformer first, with some good item handling, 2D physics maybe, then I would concentrate on that, then add the RPG elements later - but only after working out how things will actually behave in the full game, you can't really tell until you get some demos put together, nobody ever sets their game brief in stone

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