While OMD is a very cute game, it suffers from some fairly crippling design flaws. Firstly, most of the levels' difficulty results directly from one decision the devs made; instead of allowing choke-points or open spaces or a single path, the people who made this game decided that each level be bifurcated, but provided portals to allow us to traverse the two. Pick one, devs. Difficulty or ease.
As well, barricades are among the most expensive single buildables.
In this vein, it is much more economically viable to murder all the monsters yourself, as traps are A) very expensive and B) reset much too slowly. So why, for the love of Atheismo, is there only a 'cash bonus to trap kills' purchaseable!? Why can't we simply rely on weapons? Oh, right-- because the devs decided to split each level in two, so we can't simply use one route or the other. You're forced to use both the (mandatory) crossbow and (shoddy, unreliable) traps.
Still, it's a fun romp, even for its faults and flaws.

Pfffft! I was playing this game before it was COOL. It's a weird cross between a roguelike's permadeath aspects, a bullet-hell shooter's dodge-and-don't-die aspects, and a JRPG's fickle statfarming. Still! It's challenging, fun, and free (you cheapass motherfucker). Get on it.

So.
You don't own Psychonauts yet?
Didn't Yachtzee teach you ANYTHING? This game's AMAZING. 3D puzz-platorm using PSYCHIC POWERS, but primarily the reason we buy this game is the small-term brilliance of the guy who wrote Grim Fandango, among others. Please, just-- just get the game, light some squirrels on fire, dodge angry meatmen in the Meat Carnival, and have FUN with it.