In order to access Drangleic Castle, you're going to need to get hold of a handful of Great Souls which drop individual from particular bosses: the Old Iron King, The Rotten, The Lost Sinner and the Duke's Dear Freja. There is an alternative route to the caslte via the Shrine of Winter in Shaded Ruins, but you'll need a Soul Memory of a million souls in order to use it.
Here's how you can find each boss, and loot the Great Soul from their twitching corpses.
Old Iron King Great Soul
If you've not yet killed Mytha, the Baneful Queen, make sure you light a torch to burn the windmill in Earthen Peak - that'll remove the poison from around her and make the fight much easier. Once she's dead, go to the other end of the boss area from where you entered, and take the lift downwards to reach the Iron Keep. Once you've beaten this area, you'll be up against the Old Iron King.
The King likes to throw attacks at you in pairs, and so you should never be too quick to launch your counter-attack against him. You should also be able to dodge most of these attacks, so don't worry too much about equipping any damage reduction gear. If you're struggling to take him down, take a look at our complete guide to the tactics required in this fight.
The Rotten Great Soul
You'll find the Rotten by travelling down the Majula well and then making your way through the Grave of Saints. Head through this region, then go through the Gutter until you're in the Black Gulch. The Rotten awaits you at the very end of this area.
We've got a more detailed guide to the fight with the Rotten over at our boss fight page, but you should try to stick to the centre of the battle arena to avoid the worst of the fire-pit damage, and you'll also want a shield that blocks 100 percent of incoming physical damage.
You can't block all the time though, as you run the risk of running out of stamina if you do. Instead, dodge every attack you can, and keep your shield as a reserve for when you make a mistake with your timing. Counter-attack with just a single attack unless you have a weapon with a massive attack speed.
Lost Sinner Great Soul
To find this boss, make your way through the Forest of Fallen Giants, kill the Pursuer, and then use the bird's nest to get to the Lost Bastille. Now head through Sinner's Rise until you arrive at the boss area.
There are a few things to keep in mind if you want to make the fight against the Lost Sinner a little easier on yourself. First of all, make sure you're using a shield that can absorb 100% of the damage that comes your way. The Drangleic Shield is a great shout for this fight, and you'll have looted one from killing the Pursuer.
Secondly, the fight can go on for quite a while, and so you need to take things very cautiously and block all the quick-fire damage the Lost Sinner puts out. Block the damage, get a couple of your own shots in, then get ready to go back on the defensive. For a comprehensive look at the strategy required for this fight, take a look at our guide to beating the boss.
Duke's Dear Freja Great Soul
First of all, make your way through the Shaded Woods and get to the Shaded Ruins. Now kill Scorpioness Najka before entering the Doors of Pharros to reach Brightstone Cove Tseldora. Work your way through this region and you'll eventually find yourself facing Freja herself.
Unlike most boss fights in Dark Souls 2, you'll want to pretty much ignore the trash spiders that appear alongside Freja, and get stuck straight into the meatier target. Positioning is key if you want to avoid the worst of her damage, so refer to our comprehensive Freja guide if you're struggling to stay alive.
Head back to the first page for the rest of our Dark Souls 2 walkthrough.