In Cubic Castles, there are several ways to level up, like breaking blocks and completing quests. But This may be the slowest possible way to level up:
1: Collect a LOT of grass blocks (must be either regular grass or Tropical Grass).
2: Go to an Anvil or Workbench and craft all of the grass into string. ONLY craft each piece one-by-one. Clicking the Craft Max button will only give you the XP value of a single craft.
3: Now turn all of the String into Rope.
Going through these steps gives you fast XP, as mining the grass blocks, crafting them into string, then crafting the string into rope are all 3 ways to get XP in just one task.