The Imperial City Sewers are in fact not one area, but several interconnected areas that span the entire underside of the Imperial City. It connects all the major districts except the Waterfront and the Prison.
The Blades are said to use the Imperial City Sewers to move around the city unseen. At low levels, one will only encounter goblins, rats and mudcrabs. The bandits and undead will be leveled as per normal.
It is possible to cross the entire Imperial City by using the sewers. The Hero's first visit to the Imperial City Sewers is right at the start of the game. The sewers are a great place to go if one wish to escape from people/ sunlight (if one is a vampire). Overall, it is a major role in the storyline, for this tunnel system plays host to the Path of Dawn quest, and helping the Hero escape from prison in the beginning.
Entering from the garden during the day one will find a sleeping bandit and some cheap loot. For a low level character, it is a good place to practice sneaking. There is a door in the sewers that cannot be opened from the Market Sewers side. To open the door enter the sewers in the Arboretum and then go through the sewers to the Market Sewers entrance, where the door can be opened with a nearby wheel.
In the Market Sewers there are two trap doors leading to The Best Defense and The Main Ingredient. These doors both have very hard locks. Also worth noting is that if one pick these locks and are in the Thieves Guild, one can easily acquire many items worth fencing, just make sure it's after about 10 or 11 PM. The sewers contain mud crabs, rats and bandits, all with good items to loot for beginners. There are a lot of low level chests to loot.
Elven Gardens SewersEdit
The entrance is through a grate beside a well in a small garden. Going straight forward from the entrance when inside, leads to the Sewer Waterworks. Turning right and then right again after the stairs, leads to the canal. When going over the bridge in to the room next to the canal, one can find a chest with minor loot. In the same room there is an exit to Othrelos basement.
Coming back in to the canal room, turn the wheel and a gate opens in the water and one is able to go through to the next room. There is also a chest there with minor loot. Go SW along the canal until the end and in the next room there is an exit to Irene Metricks basement. There are mostly mud crabs and rats in this section.
The second part of the sewer is reached after going through the Sewer Waterworks. Walking through the canal water will lead to a room where there are exits to Sunken Sewers and Fathis Ules basement. Behind the stairs to the Sunken Sewer is a locked chest with some . Beside is another chest with some potions.
The entrance from the Elven Garden Sewers leads to a room next to the canal. In here there are leveled Goblins and they summon zombies. Near their living area one can find a few chests with minor loot. This part of the sewer has a few rooms attached to the canal. There is a second exit to the Elven Garden Sewers, this leads to the second part of that sewer.
This part of the sewer is where the Mythic Dawn members are meeting. It is empty after the quest "The Path of Dawn" in the main quest line. One can find a few chests, one is locked, with various items in them. Here are also Mythic Dawn Commentaries 1–3 found, left by the members when they left or were killed. There are two beds here one can use. The exit from the main area leads to Talos Plaza Sewers.
Talos Plaza SewersEdit
When entering this part of the sewers from the Sunken sewers, directly ahead goblins have set up a camp. There are two chests with minor loot. Walking along the tunnels and one passes the exit to Dynari Amnis basement.
Further along exits to Talos Plaza District and the South West Tunnel are found. The rest of the tunnels in this section is pretty empty except for a few mud crabs and some rats. Near the exit to the Talos Plaza is a locked chest with some loot.
South West TunnelEdit
This part of the sewer is empty except for a few mud crabs. At one end of the tunnel is a locked chest with minor loot. The tunnel has exits to the Talos Plaza Sewers and the Temple Sewers.
The Temple sewers are a large area with a round large room in the center. There are three different tunnel systems leading from this room. Leveled goblins are found here so be careful as they can be hard to dispose off. There are a few chests here with minor loot and leveled loot.
There are only exits to Amantius Allectus basement and the South West Tunnel here so one have to backtrack to get out.
The North Tunnel is divided into two sections, a small tunnel with a canal when coming from the Market sewers. Then a larger area with two large rooms and the canal in the middle.
There are mud crabs and rats in the smaller area. Bandits can be found in the larger area. A few chests with leveled loot are found near the bandits. In the large room next to the exit to the City Isle one will find a Nirnroot in the water. The exit to the City Isle has a very hard lock.
Coming in from Beneath the Bloodworks, there are two circular rooms that lead to a canal with the door to The Old Way. During the quest "The Ultimate Heist" the Hero will get a key from the Gray Fox so they can go through the palace sewer and enter the Ayleid Ruin The Old Way, hidden under the sewer. The other entrance to this part of the sewer is in the Palace. One will encounter rats and wild dog on the way.
South East TunnelEdit
There are two entrances to this part of the tunnel, one from the waterfront and one from the Arboretum. The tunnels exits to the Beneath the Bloodworks section. There is nothing of value here and only mud crabs and rats can be found.
Beneath the BloodworksEdit
In the maze of tunnels and rooms that make up the Bloodworks sewers, one will find that vampires have been known to inhabit this area of the sewers, and it is a part of the path to the Imperial Palace during the quest "The Ultimate Heist" for the Thieves Guild. There are corpses hanging from the ceiling and coffins almost everywhere. Several chests can be found with various loot.
The name refers to the area of the Arena known as the Arena Bloodworks, so called because blood from gladiatorial matches commonly seeps into the training room. It is possible that the blood that drips into the Bloodworks also seeps into the sewers below, which would explain the presence of vampires in the area.
There are exits to the South West Tunnel, Market Sewer and the Palace Sewer. A key is needed to enter the Palace Sewer as one can't pick the lock. The door to the Market Sewer has a very hard lock.
After discovering that the Amulet of Kings was taken by the Mythic Dawn during the assault on Weynon Priory, Jauffre wants the Hero to meet with Baurus in Imperial City to help find the location of the Amulet of Kings.
The final piece the Gray Fox needs to complete his grand (but unrevealed) scheme is an Elder Scroll from the Imperial City Palace itself. To start, he gives the Hero the Boots of Springheel Jak, which he believes they may need at some point, as well as the Arrow of Extrication, which supposedly should unlock something.