You can trigger this quest by finding the camp of a man named Stalleo, who is camped up near Treva’s Watch. He went off to war and while he was gone, some people came and took over his keep! He now asks for our help in taking it back as his attacks have been thwarted.
After agreeing to help you’ll be told that you must reach the gate lever to get his help. We’ll also be told to use the backdoor to get in. Nearby the camp is a cave calle "Treva’s Watch Escape Tunnel". Head on inside. You’ll be in a tunnel now with four bandits up above you. There is a fire pot trap here you can activate by shooting with an arrow, so get in a sneak attack and do that to even the odds. Head up and loot the two chests in this area, then go further into the tunnel but watch out for bear traps. Up ahead a bandit will be singing and working in a smithing area, so take him out and use the smithing stuff if you need to. A nearby room as a pile of corpses in it as well. Damn bandits. Open the nearby door to Treva’s Watch now.
Head further in and watch the floor for bear traps. Up the stairs ahead will be two more bandits to take out, so do so. Further in on the right is a bedroom with two more bandits, and if you fight too loudly a bandit chief will come from down the hallway. You can pick the door opposite the bedroom and loot the closet as well. Continue on and soon you’ll hear chatting. There are FIVE bandits of varying strength below you! Sneak over to the left edge and get some sneak attacks in, then summon back-up help if you can (as it helps a lot) and kill all five bandits. Continue through the keep through the kitchen and pantry (there’s a chest in the pantry) and out to a work area.
Past the work area you can either go straight or up. Head up first and combat three bandits up here (one was a Chief for me) but the reward is worth it: a BIG LOOT chest as well as an ENCHANTING SKILLBOOK called "Twin Secrets" on the nearby table.
Go back down and now forward. To the left is a dining/prison area (a very weird combo) with two more bandits in it. Take them out and continue down the hallway, but beware of pressure plate traps. There is a chest at the dead end, but then go to the exit. As soon as you’re back in Skyrim, the lever will be RIGHT in front of you, so hit it to have Stalleo come and support you. There are 4-5 bandits out here, so help Stalleo and his two guards wipe them out. When you’re done, talk to Stalleo and as way of payment he will give you the DETECT LIFE TOME, which grants you the Detect Life Alteration Spell. Very nice. This quest will also now be over.