The mechanics of siege warfare, both at the battle and campaign level, presented the most challenging part of the design of Domains at War. Pitched battles are glorious in their simplicity; sieges revel in complexity. Artillery, arson, assault, location, mining, supplies, treachery – all play their part.
The artillery presented in Domains at War are modeled after the very best stone-throwing weapons ever constructed. Nevertheless, the actual destruction of an enemy stronghold by stone-throwing weapons is a slow and painstaking affair in D@W, just as it was in history. Ancient artillery were very useful for shattering wooden ramparts and forcing castle defenders to take cover, but to actually breach stone walls required ceaseless bombardment over long periods of time. Most shots bounced off the stone, dealing little damage. As many as one thousand shots may have struck the same wall before the 10’ thickness of a curtain wall was breached. Each artillery piece was kept firing around the clock for days on end. At the siege of Lisbon in 1147, the crusader’s trebuchets fired 250 shots per hour for ten hours straight, a record-setting rate of fire!
Still, despite the impressive rates of fire, it will take days and weeks, and not minutes or hours, to breach a stronghold’s walls. This may be frustrating to gamers whose expectations for sieges are set by disintegrate spells. Some of you will doubtless wish to “buff up” artillery in Domains at War to achieve a more exciting pace in sieges. Before doing so, it’s worthwhile to ask yourself how much military utility you want strongholds to have in your setting. When gunpowder altered the balance of power in favor of artillery, the castle and the walled city soon became antiquated. Likewise, if you make artillery more powerful, there will be much less value in building castles and walled cities in your campaign world.
The damage values themselves were calculated based on the kinetic energy generated by the artillery. Kinetic energy is calculated with the formula k=1/2mv2, where k is energy in joules, m is mass in kilograms and v is velocity in meters per second. I had historical evidence for the mass of stone thrown by the various weapons, and velocity I approximated with v=(gd)½ , where v is velocity in meters per second, g is gravity (9.8m/s2) and d is the weapon’s maximum range. This provided a range of kinetic energy from 4,009 joules for a heavy ballista to 200,455 joules for a heavy trebuchet. An artillery weapon’s damage (average hit points) is approximately k/4.6a, where a is the area of effect in square feet. Thus, a heavy trebuchet (200,455 joules and a 20’ radius area of effect) deals (200,455/4.6 x 1,256) 34.7 damage on average, or 10d6. A heavy ballista (4,009 joules and a 5’ radius area of effect) deals (4,009/4.6 x 78.5) 11.1 damage on average – rounded to 3d6 (10.5).
You can use these formulas to create your own stone-throwing artillery of exotic sizes and strengths.
Because of the mechanics of unit capacity during assaults, having a large stronghold has both benefits and drawbacks in Domains at War. This is a feature, not a design error! Large strongholds can be defended with much larger garrisons, and take much longer to batter down with siege weapons. However, large strongholds have longer walls, require more troops to man them, and can be assaulted by larger forces simultaneously. A sprawling, under-manned castle may fall to an assault more easily than a small, heavily garrisoned fortress.
The actual unit capacity of each fortified structure was calculated by summing the number of 120-troop units that could man the perimeter and the number that could garrison the interior. The number of units that can man the perimeter is equal to the perimeter length (in feet) divided by 60’. The number of units that can garrison the interior is equal to the equal to combined the square footage of all floors divided by 2400 square feet. (Remember that a unit occupies an area 60’ long and 40’ deep).
If your campaign features new or unique fortified structures, you can use this formula to derive the number of units they can hold.
Structural Hit Points
The structural hit points in ACKS and Domains at War were calculated based on the assumption that heavier structures can absorb more punishment. Each structure generally has around 1 shp per ton of weight of external walls. Particularly well-built structures might have 2 shp per ton.
To work up shp for a structure the rules don’t cover, simply calculate the volume, in cubic feet, of all external walls. Treat each cubic foot of wood as weighing 64lb, and each cubic foot of stone as weighing 160lb. (This is an approximation, but it is based on the real-world densities of oak and granite.) Divide the result by 2,000 to convert it into tons, and this will give you a reasonable value for the structure’s structural hit points.
Since a structure’s cost is largely determined by its size, you can quickly estimate a structure’s shp from its cost. In general, wooden structures have 1 shp per 80gp of cost, while stone structures have 1 shp per 8gp of cost. There is, again, some variance based on complexity (complex structures cost more gp and have fewer shp) but at the scale of a stronghold this shortcut should be correct to within +/- 15%. This correlation allows Domains at War: Campaigns to use stronghold value as a shortcut to derive all manner of stronghold characteristics.
The above mechanics will also work for ships, substituting “hull” for “wall,” of course.