Player Characters AND NPCs go up in level by visiting the nearest adventurers guild house, or thieves guild, or warrior guild, temple or wizard guild / school.
Inside there is a chamber 4′ × 4′ in size. By stepping in and closing the door, the player is exposed to the forces of the universe and raised in character level.
Imagine Gandalf in LOTR after slaying the Balrog. Granted, being a Tolkien Wizard, he was not actually mortal but the experience and effect in this campaign is what is intended.
Multi-class level up. If a character has the experience to level up two levels, the character bounces over to the other class. Example, a multi-class druid/magic-user, 2 level druid and 1 level magic-user has declared his experience would be going into druid. While adventuring, enough experience points have been accrued to go up an additional level beyond level 3 in druid. The player has the option to place the additional experience into magic-user since they would have had the chance to switch declared class after going up the level in druid.