bolts can be found in crates, along with ammo, or dropped from defeated enemies. the player also needs to buy ammo for most weapons, but a small number can function without the need for ammo. vendors, which sell weapons and ammo, are situated at strategic points throughout levels. after completing the game, the player may choose to enter "challenge mode", in which the game's difficulty level rises considerably, but all bolts and weapons acquired the first time are carried through. there is also the option to buy "gold weapons", more powerful versions of existing weapons. the game's health system, nanotech, starts at four health bubbles equivalent to be able to take four hits, but upgrades can be purchased, giving the player a total of eight hit points.