Constable's Gate is the second hub of Deathkings of the Dark Citadel.
You begin on a high ledge looking down over several foes and the gate itself. But don't sit there idling at the scenery as there is another couple of ledge in sight with Chaos Serpents on. Note the crystal sitting on the closest ledge; the Heart of D'Sparil, which is your primary objective this hub; though of course you have to complete the entire hub to get it.
Below are several Centaurs that will be alerted as soon as you fire a weapon. There is also a single Slaughtaur among them that may take you by surprise if you choose to hang up on the ledge to pick the Centaurs off.
Once the area is sufficiently clear jump down and go to the gate. Behind it is a few Slaughtaurs. But the bigger threat here is the Heresiarch that appears as soon as you enter a little way in.
You can battle the Heresiarch or, given that you won't need to go back out, you can lure him outside the gate where he is unable to get back in if the door shuts.
Up in the room the Heresiarch emerged from is the first part of your first task this hub; collecting three Yorick's skull artefacts. Behind the skull is also where you must place the Heart of D'Sparil once you have it. There are two wooden doors leading off the grass area behind the gate. The one nearest the gate leads to a currently sealed off portal. So enter the other one. A pair of Ettins will ambush you as you reach the door. Behind the door is a large room with a switch on the back of a raised platform. Pressing it will open several alcoves with more monsters (of type varying according to your character class) inside. Several of the alcoves also contain switches. Press them all and the door to the next area will open at the far end of the room.
Cross the bridge watching for the Stalkers in the slime on either side and head up the front stairs and follow the passage around to find a valve (note the portal you can see through one of the windows). Turning it will open the Sewer gates at the far end of the slime on the right side of the bridge (pressing it again will close the gate, and again open it etc.); do this and go through the gates. There will be a narrow corridor to your right. Travel down it and through the door to a room of Afrits.
Enter the only other door to find and use two more valves. The one on the right lowers the slime in front of you while the other resets the sewer gate. Don't worry about resetting the gate, as like the other gate switch, this one is also re-usable.
Go back outside to the slime and enter the newly revealed passage to be presented with three passageways, two of which are currently sealed. Travel up the open one to find a switch which opens the bars blocking one of the other two passages. Up this second passage is a switch that opens the door blocking the third passage. Go up the lift behind the third passage to be presented with a teleporter. Do not use this teleporter; instead, go down the steps in front of you and around the corner to a portal.
The portal will take you to where you will ultimately need to place the three Yorick's Skulls, Hub 2: Locus Requiescat; but for now it leads to one of two keys you require to unseal the portal back at the gate; the Axe key.
Returning with the Axe Key
You are back, with the Axe key. Now go through the teleporter mentioned in the last paragraph to be taken to the room containing the portal you could see through the window back at the start of the slime area. Enter it to enter the Hub 2: Market Place, where you can pick up another Yorick's Skull and also the second key you need to unblock the portal back at the gate; the Steel key.
Returning with the Steel Key
Head back to the gate and use both the Axe and Steel keys to unlock the portal to Hub 2: Ordeal. Here you can collect the final Yorick's Skull.
Returning with all three Yorick's Skulls
At this point you should have all three skulls. Head to the Hub 2: Locus Requiescat to place them.
Returning with the Castle Key
Returning through the portal from Treasury
You'll appear in a new, but fortunately small area of the map. You can turn left to find a portal back to Hub 2: Treasury and also a door leading to a part of the level you've already been to.
You want to take the lift directly in front of you which will take up to a small passageway which leads to the ledge with the Heart of D'Sparil on it. Pick it up and return to the room the Heresiarch emerged from right back at the start of the hub and place it on the pedestal.
A switch will be revealed that completes the hub.
There is actually a lesser-known path for the first part of the level, which allows you to avoid fighting the Heresiarch up front. From the initial ledge, you can jump to an adjacent ledge on the right, from which you can reach yet another ledge, and then jump onto the southeast corner of the initial area.
This takes you to an area with some brown chaos serpents, which opens out to a cliff where you can jump through a stained glass ceiling, reaching the area behind the sewer areas. You can then go back out of the sewers to the bridge area, then back to the room with alcoves, which eventually leads back to the room with the heresiarch, only now you have access to the alcove room and can use it to hide from the heresiarch's attacks.
The level appears to be designed to allow this alternate route, as there are several switches that open the doors leading back to the initial area that wouldn't be there if this was not intended.