Almost all creatures in Necronia give some experience points when they are slain. The amount of experience points earned by killing a creature depends roughly on how difficult it is to defeat that creature. It varies from a Rat up to the Toxic Dragon. There are also a lot different kind of bosses in Necronia, which might wield even more experience. There are also some creatures that give no experience at all, those are no challenge to kill since they don't fight back when attacked. All characters start without experience points when they first enter the game, on a small island south of Rookgaard. By doing Lea's Test Quest they get experience, on Rookgaard they can kill monsters which give experience, which can become a very large number. The experience is represented by a characters level. On the official website is the full experience table where is listed how much experience is needed to reach a certain level.
Find out more about your experience by using the Experience Formula.