There are several places that one may train, and become more powerful. However finding them, and paying for their use can be difficult.
To enter you need: a Bonecrusher, a Firestar, and a Demonspawn Shield.
To enter you need: lich dust, a Spectre's ectoplasm, and a Skull's tooth.
To enter you need: an Ancient dragon's steak, an Ancient Blue Dragon's steak, and an Ancient red dragon's steak.
To enter you need: a Demonbane, a Belzebub's sword, and a Firebrand.
There are two minor training centers, that require no entrance fee. However there is a limit on the amount of time one may spend in them.
These are the locations to the entrances of the Training Centers.
world_106_109 - x312 - y205 - Humanoid
world_124_113 - x274 - y246 - Undead
world_107_120 - x364 - y485 - Goblin & Zombie