Bioware's challenge scaling system is meant to enable players to visit various areas of Dragon Age: Origins in a non-linear order as they prefer by automatically adjusting the levels of enemies encountered in the area to match the overall level of the player's party. However, to prevent complaints about similar scaling systems in other games, Bioware has added lower- and upper-level limits to how far the enemies will scale in any one area. The net result is that it's entirely possible to visit some areas at too low of a level and therefore find the challenge you face to be inordinately difficult because your party strength is lower level than the lower-limit of the challenge scaling for that area. Conversely, it's also possible to visit some areas at too high of a level and therefore find the entire area not even remotely challenging, because your party strength by then is higher level than the upper-limit of the challenge scaling for that area.
Ideal order you should followEdit
Although the game itself allows you to visit Denerim and Orzammar immediately after leaving Lothering, you're not meant to do so until much later in the game. Unfortunately Bioware does not make this clear at all. Going to these higher-level areas too soon is a contributing factor to some players reporting that the game is "too difficult" for them even on Normal mode. Bioware meant for the bounty hunters outside Orzammar as you approach it for the first time to be a "gating encounter" to scare you away if you're still too low level for the intended Challenge Level of Orzammar. Likewise for the Ser Landry encounter (the duel in the back alley) in Denerim. Both encounters will be doable, if not downright easy, if you come to these areas well after level 10 as intended by Bioware.
The preferred order to visit each area is as follows, based solely on the lower and upper limits of Bioware's hard-coded challenge scaling for each area. It is of course perfectly legitimate to follow a different order for roleplay reasons or because you want to add a specific companion or items to your party sooner. However, you might find that some areas are too difficult if you visit them too soon, and conversely some areas might be too easy if you visit them too late. For example, Redcliffe Village/Castle has an upper limit of level 10-11 for normal enemies and below, level 12 for lieutenants, and level 14 for bosses.
- Ostagar / Korcari Wilds > Lothering > Circle Tower > Redcliffe Village and Redcliffe Castle > Brecilian Forest > Haven > and then after all this you can hit Denerim or Orzammar in either order you prefer > then The Landsmeet and the rest of the game
Note that many players and the Prima Strategy Guide lump the Arl of Redcliffe quest arc and The Urn of Sacred Ashes quest arc together as if they were two halves of the same major quest arc, but this is not the case. Arl of Redcliffe takes place in Redcliffe Village and Redcliffe Castle, which are designed for players at level 6+. The Urn of Sacred Ashes takes place in Haven and is designed for players at level 8+. So the intended order, based on challenge scaling, is to go to Redcliffe for the Arl of Redcliffe quest arc at level 6+, then Brecilian Forest for the Nature of the Beast quest arc at level 7+, and then to Haven for The Urn of Sacred Ashes at level 8+.
For details see Challenge Scaling in Dragon Age: Origins
Highest Enemy Level and Most Challenging AreasEditThe maximum level for an Elite Boss enemy is 22 (20+2 -- possible only in areas, where the maximum level limit is 20). The areas that scale the highest are: The Battle of Denerim areas - max. level 20. Landsmeet chamber area - max. level 20. Leliana's Past world map encounter (Marjolaine's assassins) - max. level 20. Flemeth's Hut exterior during Flemeth's Real Grimoire - max. level 20.* Tevinter Warehouse during Unrest in the Alienage - max. level 19.
- Flemeth's Hut exterior (pre210ar_flemeths_hut_ext) is more of a theoretical level 20 area. The areas, as a rule, scale according to player's level at the moment they were unlocked. Since you unlock Flemeth's Hut very early in the game, the engine will use the default minimum area level (13) when you come back to fight Flemeth. Which means she will be level 15 at most. That said, doing Flemeth's Real Grimoire quest after the Landsmeet will reset Flemeth's Hut area level, thus allowing you to fight level 22 Flemeth.