The Blood of Dawnwalker allows the player to go off in just about any direction after the events of Laslea. Your time is truly your own. However, due to proximity more than anything, most players will stumble upon Mert's Mill, discover his ties to Vlad, and be tasked with saving him.
Thus begins the quest 'The Firebrand' and the follow-up quest 'Under Watchful Eyes.' A series of quests where you'll attempt to free Mert from his sell at Stoneward Post.
There are quite a few ways you can go about it. So, here is the best way to attempt it, both during the day, and at night.
During The Day
Travel to Stoneward Post, just Southwest of Laslea
This is the approach that requires the most guidance and finesse, as there are certain things you need to do to gain access to Mert without a big brawl breaking out.
First, you'll need to head to the Stonewall Post, and at the gates, a guard will ask you to part with your weapons. You should agree to do so, as you'll otherwise have to fight them.
The view from the front of Stonewall Post
It's actually best to go to the nearby shrine and leave your weapons in storage, as this means you won't need to come back and kill the entire fleet of soldiers to get your stuff back.
After this, enter the camp and head towards the Overseer's tower. You can pay a visit to the merchant and the commander, but neither is that helpful right this second.
Yenno The Merchant's Stall
If you go straight to the Overseer, the only way you'll get to see Mert is by paying a 100 Denarius bribe. But there is a better way. First, speak to the lady outside the Overseer's room, and they will tell you of a corrupt merchant stealing prisoners' belongings.
Then, once you learn of this news, head to the merchant and threaten to tell the commander of their wrongdoings. They'll then rat out their suppliers, one of which is the Overseer himself.
Discussion with the Overseer
After this, head back to the Overseer, tell them you're in on their little plan, and you'll be allowed in to see Mert free of charge.
Note: You can also go to the commander and rat out the merchant to receive a small cash reward. However, be sure to do this after gaining access to Mert, as this is your silence is your leverage to get inside.
Freeing Mert
Discussion with Mert in Stonewall Post Jail
Now comes the part where you need to get Mert out. Simply talking with him will allow you to gain access to a new quest, In The Shadows, which can be done with or without releasing him. But talking to him by distracting the guard will cost you two time notches.
Freeing Mert requires the Prisoner's Keys. These can be acquired by either killing the Jailer or taking them off the wall in the Overseer's room.
Prisoner's keys in overseer's room
The best method I found for doing this to avoid costing yourself time is sneaking to get the keys before talking to Mert. Immediately freeing him, and then beginning the fight to get him out of the camp alive.
Handy sword to pick up before talking to Mert
Before you trigger any conversation with Mert, seeing as you are unarmed, you should pick up the Weathered Sword from the bench nearby. It's not much, but it sure beats trying to break him out with just your bare fists.
From here, you'll need to fight the Jailer, then head upstairs and fight Rayko, the Incorruptible. A mini-boss fight that is just a standard soldier with a beefy HP bar. So, don't sweat this fight too much.
Boss fight with Rayko
Then, finally, you'll need to step out of the overseer's room, where you'll need to fight three soldiers and a dog.
The back exit to avoid needless combat
Provided you survive this exchange, you're in the clear, as you can head to the right of the overseer's building, and you'll find a back exit that will allow you and Mert to slip out, ending this quest.
Freeing Mert At Night
Boss fight with Cerbus
To put it simply, at night your only option is violence, and in a lot of ways, this is the better way to go about things, especially if you want more loot for your efforts. That said, it's easier said than done.
If you are heading in the front door, you'll need to fight a total of eleven soldiers and one dog before you reach the Overseer. This also includes a mini-boss fight with Cerbus, the Scornful. Plus, you'll also need to take down Rayko as you would during the day.
However, the rewards for doing this are pretty sweet, as you'll be able to pick up hundreds of Denarius, a couple of rare manuals, a Nighttime Pendant, and a Well-Made Battle Sword.
Plus, in the chest in the Commander's Tent, you'll also find a Band of Endurance, which is a nice piece of loot into the bargain.
There is also the option to skip a lot of the fighting by using your shadow walk ability to scale the perimeter of the camp and sneak into the overseer's room. Essentially mirroring the same level of violence as the daytime approach, with the benefit of vampire powers.
But the benefit of killing everyone in the camp is that this will increase your infamy stat, not to mention get the job done in much less time overall. So, it may be worth considering.