Let's start off with the end result:
The portal in the center was made by combining parts from 2 Skull Keeps with the addition of a bit of foam board on the inside and my special combination of PVA and superglue to quickly sculpt a bit of extra fire effect on the bottom.
I drybrushed a mix of Khorne Red, Wild Rider Red, Troll Slayer Orange, Flash Gitz Yellow and finally some Dead White on the flames to get the effect of fire. The glowing effect around the portal was made by drybrushing the stone to a reasonable grey and then drybrushing it with Khorne Red followed by a few more top layers of grey. The Chaos Symbol in the center got extra care and edge highlights. It draws attention and needs a bit of extra love.
Some top shots and detail shots of the rest of the building. For the curious. It was sprayed with Citadel Chaos Black undercoat and then resprayed with Retributor Armor Spray under an angle like the pre-shading technique used for airbrushing. After that I spend long hours painting all the wall coloured bits with a mixture of Dark Reaper and Vallejo German Grey basecoat (to give the Dark Reaper better coverage. All skulls where painted with Zandri Dust and drybrushed with Tyrant Skull followed by a light drybrush of Praxeti White.
To recover detail on the skulls I used a mixture of Future Floor Cleaner with a few drops of Burnt Umber and even fewer drops of Black (both Vellajo Model Air range) added. The same mixture was also used to shade the gold.
The daggers on top where painted with the same Dark Reaper/German Grey mixture as the rest of the building and the edge highlighted Shadow Grey.
The end result is a rather imposing fortress although for the price I would like to see a better fit of individual parts on it. I had to spend quite a bit of time covering up gaps between walls and sides.