Your character gets better with different game features.

Here is a list of all features that reward you with gold, XP or better attributes:

• Arena Manager: more runes give you better rune bonuses on items
• Arcane Toilet: better items in the weapon and magic shop
• Fortress:
  • Academy: generates XP
  • Gem mine: better gems per level
  • Hall of Knights: increased chance of rare gems
  • Treasury: additional inventory slot per level
• Guild
  • Demon's Portal: +1% damage for every defeated enemy
  • Treasure: more gold for quests and city guard shift
  • Guild pet: more XP for the daily missio
  • Instructor: more XP for quests
• Witch: Spell scrolls give you different bonuses, e.g. more XP for quests
• The Tower: +1% more gold per floor
• Pets: +1% bonus on the respective attribute for every pet you find; additional bonus for the following pet levels:  100 = +0,5%, 150 = +0,75%, 200 = +1%
• Mounts: reduce quest durations and change the chance to find a mushroom. Slow mounts have a higher chance to spot mushrooms at the path's edge.
• Library (Scrapbook): more XP for quests
• Potions:  Potions increase certain attributes depending on the type
• Light World:
  • Demon's Portal: gives you +1% hit points for every defeated enemy
• Underworld:
  • Gold Pit: generates gold
  • Gladiator trainer: increases criticial damage