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1 - Gareth
Gareth will tell you that he doesn't like the army much, and he'll ask you to find him a "feign death" potion so he can escape. You can buy such a potion from Father Theofolus at the church. When you give it to Gareth, you'll earn 38,000 experience, and Gareth will give you the key to the armory (#2).
If you've spoken to Isolde in the Farms area, then you can also deliver her letter to Gareth. For that service you'll receive 11,000 experience, and Gareth will ask you to return to Isolde to let her know he's okay. Nothing interesting will happen if you do that, though.
2 - Armory
3 - General Alix's Quarters
The door to the building starts out locked, but when you tell the guard next to it that you're there to deliver a message from Lord Seth, he'll unlock it for you. Then when you enter the building you'll automatically tell Alix what happened to Seth, and she'll agree to send an escort for the Aleroth healers. For completing that quest you'll receive 20,000 experience.
Note: Seth will disappear after you have this conversation, so if you've been meaning to kill him for his sword, now is your last chance.
Alix will also give you a quest to deliver a sealed letter to Captain Mitox in the Ravaged Village. You'll receive 15,000 experience upon delivery, and Mitox will give you two quests to do. Then when you finally return to Alix, you'll receive another 20,000 experience, and Alix will tell you that you've earned an invitation to Stormfist Castle to meet Duke Janus.
As soon as you finish speaking to General Alix the first time, orcs will attack the barracks. The soldiers can defend themselves perfectly well, but you might be able to land some deathblows and gain some extra experience if you're quick.
Also, by agreeing to help General Alix (and thus, in essence, joining the Ducal Army), you can access the armory (#2) and loot any of the equipment there you want. This will only last until you return to Alix after completing Mitox's tasks.