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Achaea isn't like most MMOs where you hit the level cap in a month, then spend the rest of your time playing the game trying to find something to do. There isn't a level cap in Achaea in the traditional sense. The level cap comes from how much time you're willing to invest in leveling. Before you automatically assume investing 5 years into leveling 10 hours a day will make you able to kill anyone in the game, please note that leveling has no effect on your offense and a very minimal effect on your defense.
So! That being said, what should your goal be?
You should set your goal to level 85. Level 80 is where you no longer have to sleep or eat, and level 85 is where things start to slow down a lot. Don't kill yourself trying to get level 85 in a week, instead just bash as long as you're comfortable then go do other things. 85 will give you a reasonable health pool even with low-constitution builds, which is essential for surviving against severely artifacted damage-dealers. Getting to 99 is included in this guide, but, well. Picture Dragons as the guy from the WoW episode of South Park. Not everyone is that guy, but don't expect to get Dragon just because you want it.
1-20: Minia: The good ol' days.
Your time should be spent exclusively in MInia and Lodi, learning the basic commands of the game and learning the basic quest functions. Kill imps and give them to the Pixie Queen, or pixies and give them to the Imp Lord. There are also various side-quests and other things to do, but I won't spoil it for you! Beware toxic players hanging at the archway, and there isn't really any candy in the van.
Reasonable alternatives: Lodi, Ghealdan, Ratting.
Lodi and Ghealdan are the other newbie areas, they are similar to Minia. Ratting is killing rats in any city except Eleusis, then giving the corpses to the 'rat man' of said city. Anything you do will be decent gold, so save up!
20-45: Manara: LET ME BACK IN MINIA!
Here's where it gets rough. By now, you should have your first set of armor and a way to sip health whenever it comes off cooldown (typically a system / trigger). Gnolls hit way harder, gold sucks, and there are many more people here (some people think it's good to bash Manara until 70... .... ..........) and it's reasonably tough to get to. Use WALK TO MANARA to get there if you don't know the way, but try to learn the path whenever you use WALK TO. Manara is your comfort zone, but try to find other things to kill as well.
Reasonable alternatives: Here's where your path can take you literally anywhere, you can explore and find tons of quests or smaller things to kill. You should get a minimum level of 30 before trying anything that can hit hard. Questing in Inbhir Ness is decent experience for a while, and there are dozens and dozens of areas with low-level denizens that you can kill. Make sure you PROBE things before you try to kill them.
Quick n' dirty list of small-denizen areas: Ghosts/apparitions in Belladona's Keep, questing in Actar, questing in Ulangi, dungeon of Beastlords.
45-60: Forest Watch, Goblin Villages, Orc/Dwarf Camps, small things in Dun Valley Swamp.
Kill the buckawns and give them to the dryad at the beginning of the area, but be careful because they do a lot of bleeding damage. Bring a moss tattoo and if you can learn up to clot in survival, you're a lot less likely to die from bleeding 900 (as long as you use it by doing CLOT a dozen or so times). The goblins should be your comfort zone, just kill them as you can. There are also some in upper Azdun. In the siroccians, kill the orcs and give them to the Dwarf leader, or kill the dwarves and give them to the Orc leader. Dun Valley Swamp should be bashed by anyone who has 2k+ constitution, so if you went a "tanky" build with your traits/stat pack, you can handle the animals no problem. Just bring a shield tattoo, they don't go through shield.
Reasonable alternatives: If you don't mind the possibility of being attacked by players, try the lesser things in Underworld or Annwyn. They drop good gold and are great experience at that level. Just learn your way to the exit, in case you need to get out fast. You can always quest at the aforementioned areas, and going back there is always safe if you need something to kill. Small things in Moghedu are fine as well.
60-85: Dun Fortress, Moghedu: Are we there yet?
This is where the gold starts getting good. Bring all orc corpses you get to the dwarf leader for extra gold. Don't bother trying Vukub unless you're extremely tanky. Explore and make sure you kill the whole fortress and the two orc/ogre villages. Kill everything in Moghedu excluding keepers, the Mhunna, the mhun Mage, and groups of 4+ unless you're tanky. Generally the harder it is to kill, the more experience you'll get -- but this isn't always true.
Reasonable alternatives: Kill things in Underworld and Annwyn for sure if you're European or play during off-peak hours, the experience is just amazing. Start at the areas below if you're 80 and tanky and have developed a strong hatred for either of these two areas.
85+: Meropis, Underworld, Annwyn, Istarion, Deep Sea Diving, whatever you can find.
Meropis is absolutely amazing gold/experience if you have a "glass cannon" build and are very careful. It is where I won a 4-day Great Hunt going from level 89 to 94 in 4 days as a Rajamalan Shaman with swiftcurse looping. Underworld and Annwyn should be bashed if you can handle it, but if you absolutely hate PK, hit Istarion instead. If you're extremely tanky though, you'll be missing out on the Death Knights, which are the best experience/gold in the game. Annwyn's Sidhe and Unsidhe Courts are extremely good experience/gold as well; but again, it's balanced by the fact that anyone can come down there and smite you.
Hope this helped!
Keep in mind that this list is by no means exclusive and if I listed every single area that was ideal to level in at every single level, this would be a good 14 or 15 thousand word guide. Achaea is a very beautiful game if you read the descriptions, which can help alleviate the repetitive feeling bashing gives you. Explore and find new things to kill if you ever start feeling like bashing is becoming a job, regardless of what the guide says. There is no wrong action. There is no wrong way to play Achaea. If you want Dragon, set a long-term goal. If you want level 85, set a long-term goal. The most important thing is not to burn yourself out.