Treva's Watch is a dungeon in The Elder Scrolls V: Skyrim
Before you begin any quest or explore the Watch you should find Stalleo and his 2 bodyguards to the East. This is also the safest way to get into the castle (aside from sneaking in the back). Talk with Stalleo to pick up his local quest to complete with whatever you are doing at this location.
Slipping in through the nearby entrance is recommended. As you head down the corridor, you will encounter a few Bandits in this first room. They are waiting at the top of the nearby steps. Try to pick them off with sneak attacks. There is also a lamp that you can shoot to to quickly take them out or at least heavily damage those three. There is another Bandit in the next room as well. You can find some chests and some Soul Gems.
Move into the next hallway and you will find a bear trap sitting on the floor in it. Avoid it and then head through the hallway. At the end of the hallway you will find a bandit working at an Anvil. Get him with a sneak attack to ease things up. From there go through the nearby door. You find the corpses of 3 people: 2 Nords and an Imperial. Whoever they are, they are not identified.
Enter the door to get into the dungeon of Treva's Watch proper. There are 2 bear traps just inside the first hallway around the turn. Move through carefully. At the top of the stairs you will spot two more Bandits. Try to engage them at a distance and lure them down the stairs. This will save you from getting noticed from the nearby additional bandits. This includes a Bandit spell caster and a Bandit Chief which are better taken on separately. This is a very hard fight. If you want to avoid it, there is a room to the right as you head up the slope. Deal with the 2 bandits inside. On the shelves closest to the door you will find an Invisibility Potion. Use it to vanish and just run past them (preferably after you cast Muffle on yourself. It helps to also to lay some traps and then lure them in.
If you have yet to find the Bandit Chief, you will find them as you head around the corner. Try to trap as much as you can or ambush your target. Take him out and you will find Spell Tome: Bound Sword. This is the hard part of the fight, especially if you have the Spellcasters backing it up. From there it is pretty simple to make your way through the rest of the fortress. You will pass through a kitchen area with a lot of food and a number of cooking spits and pots that you can use to make something. You will encounter a boss after you ascend to the heights of the dungeon.
Past them, it is fairly straight forward to make it all the way through to the stairs leading to the tower. There are only a few bandits (with a leader and a spellcaster at the top) on the stairs. Take a little time to loot the big chest at the top of the stairs. This lets you grab any quests items you were sent after. take a moment to climb the nearby ladder as well. This leads to another chest sitting on the top of the tower overlooking the area. After that, just head down the stairs and then out the front door. There maybe a bandit or two more to deal with, but overall, easy to get out.