Location

Sellia, Town of Sorcery is found on the shores of the Swamp of Aeonia, right in the bloody heart of Caelid. There are three ways that you're likely to come across Sellia during your travels:
  • Travel into Caelid via the northern path in Limgrave. If you follow the long, curving road south it will eventually bring you to the southern entrance of Sellia.
  • Travel into Caelid, then go through the Swamp of Aeonia. This is a more dangerous, but much shorter route. Sellia is on the northeastern end of the swamp.
  • Use the teleporter chest in Limgrave's Dragon-Burnt Ruins to jump to the Sellia Crystal Tunnel. Leave the tunnel and you'll be on the shores of the Swamp of Aeonia. Sellia is north of where you come out.

Sellia, Town of Sorcery

Sellia is populated, unsurprisingly, by a variety of magic users, all of whom are invisible until you get close enough. You may not know anyone is nearby until a Glintstone Pebble comes flying at your face. Be cautious when moving around the town. There are also some Marionettes in Sellia which are, mercifully, always visible. You can summon Spirit Ashes at any point while exploring Sellia to offset some of the danger.

Because Sellia has multiple avenues of approach it's a bit difficult to decide where to begin. Rather than providing a step-by-step guide to finding all the items of interest on the grounds of Sellia, we'll instead proceed in bullet-point fashion. If at any point you need to restore your health you can use the Sellia Under-Stair Site of Grace, at the north end of Sellia and down by the edge of the bog.
  • On the south end of Sellia is the Sellia Gateway. Aside from a single Giant Dog that stands watch in this area its only occupant is Gowry, an aged sage who wants a favor of you. Helping him is a part of Millicent's quest, and if you find an Unalloyed Golden Needle for him (gained by defeating Commander O'Neill, a boss out in the Swamp of Aeonia) Gowry will give you Sellia's Secret. Proceeding through her quest will also turn Gowry into a Sorcery vendor.
  • By the gate into the main part of Sellia, Town of Sorcery, near Gowry's hut, is the local Summoning Pool.
  • If you walk down the 'main' street of Sellia from the southern gate you'll find a cluster of rubble on your right as you walk between the buildings. Among the rubble is the "Redmane" Painting.
  • Along the topmost street, before you start hitting the rock walls behind Sellia, you'll find two Poison Greases. They're at the top of some heavily-guarded stairs.
  • Just north of the Site of Grace are the ruins of a house. Inside them you'll find the Rotten Stray Ashes, protected, predictably enough, by a dog.
  • If you climb the stairs in the north of Sellia you'll find a tree with a Golden Seed at its base.

That's all for the items on the ground. Next, check the rooftops. There are a number of ways to get onto them. Note that you can use Torrent here, allowing you to make tricky jumps between buildings much more easily.
  • You can jump from the lower railings to a house on the edge of the swamp. On its roof you'll find some Poison Blooms.
  • There's a root growing out of the stone walls beneath the golden tree where you got the Golden Seed. Jump onto them, then over to the next long building to the south. There's a Staff of Loss sitting on a balcony on your right.
  • Behind the same building where you got the Staff of Loss is a branch with a Scarab sitting on it. You can use Torrent - or a ranged attack - to reach it and take it out. This will earn you the Double Slash Ash of War.
  • Around the middle of Sellia is a lower house that you'll need to leap off of a branch to reach. Dangling off the ledge of the roof is a body that carries a Toxic Mushroom. Get up to the upper section of this same building, then double jump your way north to reach a Cerulean Tear Scarab.
  • On the south end of Sellia you'll find a thin archway that connects two sets of buildings. Laying on the archway is a body that holds a Stonesword Key.

The Sealed Doors

As you explored Sellia you'll doubtless have noticed two things: 1.) There are blue energy seals blocking sections of the town, and 2.) There are towers with braziers in them. Each time you light a flame in one of these braziers one or more of the sealed doorways will disappear, permitting you access to whatever lays beyond. The towers are in the following locations:
  • Across a branch from the golden tree on the highest level of Sellia
  • Across several gnarled branches on the southeast side of Sellia
  • Across the archway mentioned earlier, on the southwest side of Sellia (and easily accessible after lighting the second brazier) 
As for the broken seals:
  • The first is north down the street from the "Redmane" Painting. The chest beyond it contains the Night Comet Sorcery.
  • The second is along the railings on the west side of Sellia, overlooking the swamp. The chest beyond it contains a Spelldrake Talisman +1.
  • The third is along the eastern side of Sellia, behind the row of buildings. The chest beyond it contains an Imbued Sword Key.
  • The fourth is by the golden tree. The path beyond will take you out of Sellia, and off to meet the girl of whom Gowry spoke earlier.
  • The fifth and final is on the northern outskirts of Sellia. Behind it is a boss fog. Battle time!

Nox Swordstress and Nox Monk

A pair of glorified normal enemies, the Nox Swordstress and the Nox Monk are not a big deal so long as you can keep them separated. The Swordstress uses a bendy blade that can hit you at great distances, while the Monk uses a blunted mace that... can also hit you at great distances. Some tips:
  • Both enemies move towards you slowly throughout the battle, though the Swordtress is much more aggressive and will spend more time trying to engage you. Separate her from the Monk and take her down first. A good shield can ward off most of her attacks.
  • Both foes have deceptively long-ranged weapons that can strike you long before you're standing toe-to-toe with one. This also means that once you move too close, however, they can have a hard time hitting you. 
  • Melee attacks work better to take out the Swordstress, while ranged is a bit easier on the slower, heavier Monk. If you can use both styles of attack you shouldn't have any trouble with this fight.
  • Summoning is always an option, and will trivialize this battle. Let your summons pick on one enemy while you handle the other.
You'll receive 6,400 Runes and the Nox Flowing Sword for defeating the Nox Swordstress and the Nox Monk. You'll also gain access to a chest that contains Lusat's Glintstone Staff, one of the best staffs in Elden Ring. (Though given its high requirements, you probably won't get to use it for a while.)

Defeating the two Nox warriors will bring your time in Sellia, Town of Sorcery to an end. If you plan on following through with Gowry's quest, take the unsealed pathway near the golden tree to continue your journey.