The Landsmeet is proposed by Arl Eamon of Redcliffe as a way to deal with Loghain's treachery, by putting him up against a vote of his peers. (This is after gathering all armies. Once together, Eamon will allow for the Landsmeet to begin.)
Upon arriving in Denerim for the Landsmeet you are informed by Queen Anora's handmaid that the Queen is being held captive by Arl Howe in his estate in Denerim. She requests your assistance in freeing her. You must go to Eamon's room for this scene to start. You need to sneak into Howe's estate to rescue Anora. Note: after defeating the four guards who begin to follow you onto the estate you will automatically gain the armor needed to sneak into Howe's estate.
Arl of Denerim Estate
This Estate is far more than just the place where Anora is held captive. It is in fact the only place to gain supporters against Loghain (outside of the Alienage) and is therefore the only thing that will ensure you win the Landsmeet. Your chances of winning the Landsmeet are begun in positive and negative support from the minute you arrive in Denerim and how you structure your quest to Arl Eamon's request to look around and see if you can garner support. The first place you need to go is Arl Howe's Estate.
You have to sneak into the Arl's estate by procuring guard uniforms (which you will be automatically changed into once you have them). Note: You cannot have Shale in your active party during this quest. If you do, before you move to sneaking into the Estate, Anora's handmaiden will request Shale be replaced.
Inside, you are free to explore as much as you like, and nobody will question you as long as you keep the uniforms on. Once you reach Anora's door, you will find that a mage has put up a forcefield on the door. You need to find the mage, who Anora says will likely be with Arl Howe, to disengage the forcefield. Keep going down the hall. If you have not finished exploring the area, if you enter the first room in the last hallway do not go too far into the room (though it is possible to go far enough into the room to gather codex), you simply need to allow him to get to a point in the conversation with his female companion that she begins talking at which point he will be sufficiently distracted enough to allow you this far, but do not go any closer to him because unlike the other guards, this one will not ignore you completely. He will realize you are not part of the regiment, and will sound an alarm. This will cause all the guards in the estate to turn hostile.
As you make your way through the Arl's dungeon, you will come across many people imprisoned there. The very first one will be Riordan, another Grey Warden who will become key to the plot further on. He is also the person you will need to talk to in order to find Duncan's Shield. You need to ensure you get the 'Grey Warden Papers' from the chest in the room before you enter the cellar. These are the papers on the joining ritual and Riordan may need them later.
Rexel, the missing soldier for the Chanter's Board quest is down here. Two other notables are also here, both relatives to Banns attending the Landsmeet. One is tied to a rack in a separate torture chamber and the other is in one of the cells behind Arl Howe. Once you release and speak to them, their family will be waiting at the Gnawed Noble Tavern. The former Arl of Denerim's son is also locked up but, providing you have released the other two, his help isn't necessary. If you tell him that your rescue will be very costly, he will give you a key to a chest containing 40 sovereigns. With a high enough Coercion, you can take the key and let him rot. Releasing him on top of the others nets you a very high approval in the Landsmeet, but it isn't necessary. Additionally, you will find Soris (provided you are not a City Elf who chose not to implicate Soris) in a cell near the former Arl's son; releasing him makes Shianni more receptive to you in a later quest (and nets you +4 Zevran approval).
Howe himself is found in a dungeon room with two mages, one of who can shapeshift. If you have a mage with "Mana Clash," now is a good time to use it; other wise, just use the usual strategies of keeping
Once the battle is done, check the area behind him to free/not free the prisoners, then return upstairs to Anora's room. Anora's response isn't one you want to hear when you're about to decide who to give the crown to, just remember Arl Eamon's warning. As you make your way to the frontgate, you will be accosted by Ser Cauthrien and a large number of Loghain's guards. You can either surrender or put up a fight. If you surrender, you will have to fight her later on anyway, so you don't get to skip fighting her and you don't risk missing her loot either way; the choice is yours. If you surrender or fall in battle you, along with Alistair (if he is with you), will be taken to Fort Drakon where you get to play an optional quest, Captured!
Note: If your lockpick skill is high enough, or another rogue in the party has a high lockpick skill, there is a really nice Kaddis and collar for Dog being kept in the Dungeons in a locked chest in the warhound room.
Vaughan is a possible individual for release in the dungeons (unless you are playing as a City Elf and killed him). There is a possible bug that can occur when you approach Vaughan's cell (as an individual other than a City Elf) which will cause Vaughan to be behaving as though he's awake, but if you attempt to speak to him, all you'll get it snoring. You're also incapable of opening Vaughan's cell as usual. Meaning your are unable to garner his family's support in the Landsmeet. This can be remedied by taking the door to the first floor, and then going back into the dungeon. At this point he will react as normal. Note also, if you do release Vaughan, you will net -4 or -5 approval points from Zevran who seems to be the only companion who reacts to the release of prisoners in the dungeon.
If you mention he is the arl's son the elf mentioned, he will offer you the key to his lockbox containing 40 sovereigns, not bad. You can persuade him to hand you the key first, and choose to leave him in the dungeon or kill him for his crimes after.
Although Vaughan can be released in exchange for his support in the Landsmeet, his vote is not necessary to win it.
Battle with Ser Cauthrien and Loghain's Guards
If you're determined to go through with this fight, pull back as soon as you can into a corridor or room to force the archers to chase you down, gather together and provide an easier target for mass crowd control. If you retreat fast enough and far enough back into the corridor you came from and into the first room on the right (further down the hall and opposite the side from Anora's cell), it is possible to lure Ser Cauthrien by herself into the room, where she can be dealt with ease. Her remaining guards are much easier to handle once she is out of the way, and they can be taken care of at your leisure. Should you manage to beat Ser Cauthrien and the Loghain's Guards, loot the Summer Sword from Ser Cauthrien, a very nice two handed sword.
Should you fall or surrender, see Fort Drakon for information on the release/escape.
It should also be noted that surrendering or losing the fight with Ser Cauthrien is not the last opportunity to obtain the Summer Sword. Following your escape from Fort Drakon and the quest at the Elven Alienage, Ser Cauthrien will again attempt to stop you just before you enter the Landsmeet Chamber as the Landsmeet is about to begin. Here she is a much easier target, having significantly fewer guards at her disposal.
After Fort Drakon
Once you have escaped or been released from Fort Drakon, you have the chance to make good on getting people to side against Loghain. Everyone you need to find who has had a son/brother imprisoned, can be found at the Gnawed Noble Tavern. Give the sister the ring if you want her support. If you do not give her the ring, you will lose her support at the Landsmeet. There are several individuals who will give their say at the Landsmeet, one of them will always vote for Loghain, you need to procure the support of the others. Don't forget to approach the lone man in the booth closest to the door in the Gnawed Noble Tavern and talk to him to ensure you secure his vote.
The Alienage will be in turmoil when The Warden arrives: apparently a plague is going around, and some Tevinter Mages have been "curing" the sick, although Shianni is convinced that something is afoul. Go around the back of the hospice and use the back door (you will need to either bribe or fight the elven guard) to get inside. Fight the guards inside, and inspect the pile on the table to retrieve a key, coins and a note. (Not sure on this.) Talk to the elves in the cages to get more information, then head back out (through the front or back, doesn't matter) and continue on to the Apartments further down the alley. There will be more Tevinter guards/mages to fight; after the battle, continue on through the door to another back alley.
There will be another set of guards to fight, after that, head into the warehouse. You may choose to either fight or persuade/intimidate the elven Tevinter guard, Devera; persuading has her take you to the head of the operation. This Tevinter mage is willing to cut a deal with The Warden, but only if he is allowed to take the elves with him. Note that any "good" characters will disapprove if you decide to let the slavery continue. Either way, you will need the "Slave Papers" from the Tevinter Mage in order to use the information against Loghain in the Landsmeet -- you can retrieve these by either dealing with the Mage or by simply killing him. If you did not kill his guard earlier, she will join the fight now.
After all this is over, head back to Arl Eamon's estate and show him the Slave Papers. You have to do this if you plan to use Loghain allowing/supporting slavery as evidence in the Landsmeet. If you do not show the papers to Eamon, this will be considered a false unsupported claim and will work against you.
The Landsmeet Decision
If you didn't kill wipe out Ser Cauthrien's troops at the Arl of Denerim's estate, she will attempt to block your entrance to the Landsmeet. You have several conversation options here with Ser Cauthrien which makes a second fight completely avoidable. If your persuasion skill is high enough she will even admit to knowing that Loghain must be stopped and she will allow you by without incident.
If you complete all the quests you can for the nobles, you will win the Landsmeet, but you cannot use evidence or expect support if you have not followed through. Meaning you have to have gone to all the nobles in the tavern and showed Arl Eamon the Slave Papers.
At no point during your negotiations should you ever say anything about "Having left King Cailian to die". This will gain you negative approval points in the Landsmeet and will always lead to your defeat. If you have followed through, gained support and set out your points according to those accusations you can support, you will win. However, if you make a decision before the Landsmeet that you will not support Anora, she will betray you in the Landsmeet. As David Gaider has been quoted to say, it is important to remember that Anora is her father's daughter.
If you have not completed enough actions to gain the support of the majority of the nobles, you could lose or the Landsmeet may not reach a decision -- the votes being tied. If this happens, Loghain will order you arrested. A fight will break out after Arl Eamon orders his men to back you up; Anora's royal guards will also help you against Loghain, but only if you have supported her for succession to the throne. After you deplete Loghain's health, the nobles will break up the fight, and suggest that the matter be settled with a one-on-one duel.
If you win the Landsmeet, Loghain will refuse to give up power, and you can challenge him directly to a duel without fighting the larger battle. You can choose to fight him yourself, or have one of your companions fight him (except Dog; choosing him as your champion nets some amusing dialogue, however).
When you win the duel, Loghain will surrender, Riordan will appear and you will be given the chance to show Loghain mercy and induct him into the Gray Wardens. The induction can occur only if Anora is supported as queen and someone other than Alistair duels Loghain. This decision has consequences that can greatly alter the story.
Alternatively, you can choose to kill Loghain, or let Alistair kill him. You will then be given the decision on who will take the throne: Alistair, Anora, or have them marry and rule jointly. You can also marry Anora and become her consort under the right circumstances. After you make your decision, the person you chose will order the nobles to mobilize their armies, and the Landsmeet will adjourn. If you have chosen to allow Alistair to kill Loghain, Anora will refuse to marry him, taking that option away from you.
The Heir to the Throne
You can either attempt to persuade Anora and Alistair to marry (if Loghain was killed by you as Anora will refuse to marry the individual who killed her father), put Anora on the throne, or put Alistair on the throne. Alistair will initially be upset, but you will not lose many points with him. If you have hardened his personality through the Alistair's Family quest, he will be far more willing to take over rule. Once he confronts you after the Landsmeet, you can actually gain the point you lost back, provided you make the correct choices during the ensuing dialogue. If you are a human noble and female, you can propose that you and Alistair rule Ferelden together. If you should spare Anora being executed, she will admit that she would not have shown such mercy to Alistair.
It should be noted that if you did not harden Alistair's personality as above and you are romantically involved with Alistair but not a human noble, he breaks up with you, telling you that he needs to have a child and that you cannot be his queen. If you have hardened his personality as above though, it is possible to continue your relationship with him, though you cannot request that you rule together as you are not nobility. It is also notable that if Alistair has a poor relationship with you, he cannot be persuaded to marry Anora.