Advanced classes will become available once your character gains a few levels. Since Ground Zero is more of a 'short-term' game, the advanced classes have five levels instead of the typical ten levels, and characters will be eligible after as few as three basic class levels.
The following advanced classes are currently available:
More will be added in the future; possibilities include Psion, Tribal Defender, Skulk, Juju Doctor, Nanosmith, Beastmaster, Bowmaster, and Forsaker.
Most PCs will eventually choose an advanced class. Characters can gain levels in one advanced class without restriction, but each additional advanced class incurs a cumulative 20% XP penalty. Many advanced classes also have additional multiclass restrictions.