The enemy scaling is based on the number of rooms you have discovered that have a Monster encounter inside of them. The more you explore, the stronger wild Monsters become. This also means that, if you missed a path in one of the earlier areas, and later come back to it, the Monsters there will be at your level.
Also note that once a level is assigned to a room with monsters, they will stay at that level on that savegame. The idea of this system is to have similar experiences independent of the order the player explores the areas. (once you complete blue caves you are free to either explore stronghold dungeon, ancient woods or snowy peaks first). The level of your own monsters is not relevant for the enemy spawn levels.