This article is based on a discussion between the above mentioned persons on the Direwolf mailing list on what a Skaven Castle should look like and how the Skaven would Defend it.

Skaven don't need to defend much in a siege. Sure the "castle" can be defended but its only an outpost and the entry tunnel have been designed to collapse so it will be hard to follow the Skaven underground. Most assaults against Skaven strongholds are conducted not to occupy, but to destroy. Besiege the stronghold, kill all the Skaven and destroy the underground facilities.

Why a castle
So the first question I have to ask is why would the Skaven ever build a stronghold above ground. If I knew that, I'd have a better idea of what one would look like. Then, what possible scenario there could be for a Siege on a Skaven stronghold?

Most of a Skaven's life is spent underground, in tunnels, sewers, caverns, and only occasionally do Skaven venture aboveground. Either to wage war or to conduct secret missions of poisoning, assassination, and so on. Therefore, forts and castles for the purposes of guarding borders, storing supplies, garrisoning armies, protecting peasants, and so on, are largely unnecessary to Skaven.

Sometimes a factory cannot be built underground, a surface factory is needed. Skaven Doomwheels are too wide and tall for the underground tunnels! These factories have to be located on the borders of Skavenblight, otherwise they would sink into the marsh if built too near the interior. This makes them vulnerable to attack so they need to be fortified and protected.

Warpstone refineries can function perfectly normally underground, but they generate excessive amounts of noxious gases and liquids. Even if they don't affect Skaven they must still be vented above ground. However in doing so they serve as a notice to any intelligent race that Skaven are operating in the vicinity. In such a case the Skaven might build a fortress around the main aboveground entrance/exit/exhaust vents, so that opposing armies couldn't just roll some casks of gunpowder down the exhaust vent and blow the factory to pieces.

The "castle" is a former human or dwarf castle, outpost, hunting castle, fortified brewery or whatever small structure. The "castle" have been attacked and overtaken by the Skaven. While a large structure would be too important to the owner who would mount attacks on it, a smaller structure can be considered lost as it hold little strategic value. The Skaven have simply moved in and have yet to be thrown out.

Skaven have good cause to build at least modest strongholds in swamps or plains to grow crops (like the black cornfields near Skavenblight) to feed the truly incredible numbers of Skaven below ground. They are food sources below ground, not to mention cannibalism, but the food is scarce and life is grim in the Skaven life. Perhaps heavily fortified and well hidden fortress farms grow the crops that the truly influential and powerful Skaven glut themselves upon while their lessers starve, as well as stockpile the grains that will feed their personal armies when they fall upon hard times.

Occasionally Skaven must venture, and even build, aboveground because building underground is too impractical for one reason or another. The ground may be too wet to build easily underground or the area is too rocky.

The Skaven fleet has to have harbors, dry-docks and protective enclosures. The fleet will be vulnerable to an attack and need to be protected.

Some Skaven activity requires quantities of fresh water, either as a raw ingredient or as a power source, so they build a fortified dam or millhouse on a nearby stream.

Food supplies may need to be stored aboveground. Perhaps the ground in a given area is too damp, and food rots if stored underground, so the Skaven build fortified warehouses and granaries in the area. Crops and livestock may have to be raised that require sunlight and/or fresh air

Inherited castle
Metal and stone pipes connecting different parts of the fortress, ramshackle shacks tacked one atop the other, ladders leading to higher levels, and the whole thing looking like a good stiff wind would blow it all down.

Skaven often occupy enemy castles after they've poisoned or slaughtered the previous occupants. You could just do a plain old fortress for any other race and paint on some Skaven symbols. But that wouldn't be very Skaven looking. As stated in the Siege book, the bulk of a Skaven fortress is underground.

There are in fact two kinds of Skaven castles the first is a low castle. The center of the surface constructing will be the main tunnel leading down below. The entrance may take the form of a mound, a building or a crude rock construction. Around the entrance will extend a trench-systemtype with walls built out of materials at hand such as sections of old walls, logs or haphazardly stacked bricks. Among these will be bunker-like emplacements. However this looks more like a WWII trenchwar then a castle, perhaps a single center tower or tall building but not a fullblown castle.

Lots of pipes for them to scurry through! Look at he Doombringer model from Man O War - That's pretty much how I envision large scale Skaven constructions.

The second castle is a full or high castle, it is an existing castle that have been overrun by Skaven. Barricades have been created by collapsing structures so the walls are protected at the base by difficult terrain. These barricades have also been dug out to form trenches and include tunnels or small holes where ambushes can be laid. Of course some lengths of the barricade are not suitable for digging in and have had fake tunnel entrances and traps set in them. The walls themselves are in a ruined state after decades of neglect. The walls are lined with balconies that function as battlements and tall wooden ramshackle towers house Clan Skryre snipers. There would also be several gates, or rather openings, tunnel entrances and large passageways. These may be barricaded and can sometimes actually have some form of gate. Connecting everything would be ladders, poles, ratwalks, tunnels, and pipes for the Skaven to move around with, in addition they would provide many ways to sally forth behind enemy lines or escape attackers.

Skaven architecture
Skaven are marginally better craftsmen than Orcs, but much worse than Humanworks.

The outer defenses will consist of lots of dirt, sandbags, random stones, whole logs, barrels, boxes, haphazardly stacked brick and stone walls, rough wooden walls and beams. Much like the bunkers of WWI trenches or Vietnam revetments.

Features of Skaven architecture:
Lots of carved and painted Skaven symbols
Lots of spikes and spears planted point-outward in the walls
Impaled enemies
Rickety ratwalks joining sections of trenches and the central keep
Lots of rats scattered around the fortress.
Gleaming white marble that is cracked, pitted, & blackened
Methane-burning cressets
Huge solitary buildings
Narrow slit windows
Mills under warpstone wheels driven by slaves (Skaven, human, and other)
Lines of old walls and arches still visible
Broken houses
Shattered halls
Cracked & tilted paving stones
Pocked with dark tunnel mouths
Gouting flames
Foul vapors
Pale lights in the tallest buildings
Monstrous belfries
Assymetric buildings
Many different architectural styles and stone colors
Skryre-lightning contained in glass spheres on poles
Forges, labs and workshops
Mine railways and treadwheel elevators
Not to mention lots and lots of rats and other vermin

Modelling tips
Grendel does a number of beautiful resin ruins, including a Ruined Cathedral, Ruined Acropolis, Ruined Keep, Aztec Ruins, Ruined Gothic Archway and Ruined Greek Temple. Armorcast also does some very nice Ruined Cathedral pieces, bone walls and trenchworks.

For black cornfields, Dixon makes some great corn plants. I don't think Skaven would be all that neat so I wouldn't "plant" the corn in rows, but rather in clumps and singles scattered around a board, as well as some random giant rats, some ubiquitous mushrooms and maybe a skull or two. Or what about rice paddy-like conditions with flooded fields surrounded by low dikes, then surrounded by the walls and trenches. There are also lots of O-scale railroad and 1/12 scale dollhouse fruits and vegetables available. How about making some fields of giant mutant veggies, with appropriately bizarre colour schemes.

The trenchwar look can be made with modular trench sections (probably 12"), wall sections, 6x6" or 12x12" fields, bunkers, "nicer" buildings and tunnel entrance. Make each separate and they can be mixed-and-matched for many Skaven battles, even non-Siege ones.

One alternative would be to make hollowed out hill formations, somewhat like barrows with lids that could be taken off to reveal the models inside and with holes and hatches in the hillsides that would allow access and fire from. You would have to plan how the whole thing would look very carefully, but construct it like a modular terrain project. first deciding on a size and getting foam layers to represent the base and gluing the interior wall plan to the boards. Then layout the styrofoam interior walls once you have "cells" of appreciable size and shape with connecting tunnels you can build up the outsides of the walls with either textured filler to add a slope or spray-can expanding foam, which you can later contour into a sloped hill shape. Once that is done a bunch of boards which look like they have several interconnected decapitated igloos, make sure the hills are tall enough to comfortable store models with the lids on or off. You can make tops for them with other layers of carved polysterene. Flock the whole thing and decorate to taste, maybe even putting crags or trees on the surface of your "hidden fortress".