Name: The Siege Battle AI of Total War: Warhammer (Part 5 of 5) | AI and Games
Uploaded: 2018-12-16T06:23:02.000Z
Duration: 13 min 46 s
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Hi I'm Tommy Thompson, this is AI and Games
and welcome to part 5 of the AI of Total War.

In part 3 and 4 of this series I explored
the pivotal changes made to the campaign starting

in 2013's Total War: Rome II, followed by
2015's Total War: Attila. If you recall way

back in part 1 of this series: the three key
AI systems that drive the game are the unit,

battle and campaign systems. Each of these
are distinct from one another and handle distinct

parts of the core game design. However, there
are a significant number of subsystems that

exist alongside or within these three segments
that are critical to the games function. In

part 4 of the series I talked about the diplomacy
systems and how they largely operate, with

some of the more recent innovations that arose
in Total War: Attila. In this video, I'm going

to tackle a pretty important sytem one I haven't
got around to talking about yet the: siege

battle systems. Siege battles are large-scale
attacks on fortified locations and structures

that are critical to a players hold of a region
in the campaign. While sieges have been in

Total War in some form or another since the
very beginning, I'm going to look at a more

recent entry where the system was rebuilt
to accommodate it's grander aspirations: Total

In its 10th major release, Total War moves
away from history and into fantasy. More specifically

Games Workshop's long running Warhammer Fantasy
franchise. No longer tied to periods of human

history, Total War: Warhammer offers a diverse
and colourful cast of factions including the

human faction the Empire, the Greenskin faction
comprised of orc and goblins, Dwarfs and the

Vampire Counts who command the undead. Not
to mention the The Chaos faction and human

Bretonnian faction also made available as
DLC. Total War: Warhammer makes some big changes

to combat: each faction has access to new
hero units, special units such as giants,

flying units and a spell system which can
have impact both on the campaign map as well

as the combat sequences. This is combined
with a mixup in the makeup of each faction:

each with a unique configuration of unit types
and abilities. This really shakes up how a

traditional Total War match might play out,
given the Vampire Counts don't have access

to ranged units, while the poor Dwarfs are
too short to ride horses!

While these changes influence combat gameplay,
a lot more was happening in how Total War

manages sieges: massive battles as players
attempt to capture or defend a walled city.

Siege battles carry large stakes on the campaign,
given if the attacking team are successful,

it will allow them to assume control of a
province on the map. While they've been a

mainstay of Total War for several years now,
Creative Assembly took the time to rebuild

the AI systems for Warhammer not only to enable
them to handle the new capabilities of factions,

but also to make them more extensible and
manageable. Let's lay siege to the city and

A siege battle begins when the attacking player
sends an army to attack the capital of a given

province. Before actually beginning the siege
battle however, they can take turns to build

siege equipment such as catapults or siege
towers to allow them to attack or mount the

city walls. As battle commences, the attacking
army is positioned just outside the city,

with defending garrison forces and any reinforcement
armies lying in wait either atop the city

walls or on the streets. An attacking armies
goal is to gain access to the city itself

by either breaching the gates, mounting the
walls or destroying them outright. Once inside,

the attackers must push through and capture
the victory point on the map: typically within

Previous entries in the city, notably Total
War: Rome II and Attila, focussed on creating

sieges that felt historically authentic: first
bombarding cities to weaken or destroy defences

from all angles, prior to moving to attack.
Even until recently, the implementation was

a relatively simple single tactic control
system governed by finite state machines,

a topic I covered way back in my exploration
of Batman: Arkham Asylum. But this time, the

Warhammer team wanted to craft something that
was fast, intense and largely focussed on

a single direction of attack. As a result,
the system needed to learn how to execute

numerous actions in parallel, as well as how
best to utilise many of the new special tactics

But before we do this, let's take a peak at
what the combat AI systems now look like,

so as best to understand how the siege systems
operate.

Back in part 1 of this series, I talked about
how Total War uses a main combat AI system

for it's in-game battles. These systems implement
behaviour akin to player decisions and move

units of a given army around the battlefield
to complete objectives. But how does the AI

go from making a decision about an action
all the way down to executing it? After all,

the series is much more complicated now than
it was before. The combat AI systems utilise

what is known a Battle Model system as shown
here. The focus of this system is to take

a given alliance of one or more armies and
break down a high level strategy into small

and manageable tactics that can be sent to
individual units. High level strategy is set

by the Grand Tactical Analyser or GTA, that
decides whether the entire alliance should

attack or defend at a given point. This is
then broken down into objectives that need

to be executed to achieve a desired strategic
outcome. Objectives are then passed down to

a detachment of units and a tactic is then
assigned to them to tell them exactly what

So when in combat, the key thing is what tactics
are selected in order to achieve the objectives

set by the Grand Tactical Analyser. In standard
combat, we might be looking to advance in

formation, to hold a position, to flank an
opposing detachment. But when attacking in

siege combat, it's broken down into three
key tactics:

- Assaulting the city gates
- Scaling the city walls

There are also two additional tactics the
Siege AI systems need to utilise:

- The storm tactic: which is only achieved
once a certain number of entry points are

- The Reserves tactic: a special ability,
in which the AI will not commit all of its

forces to an attack at the beginning. Instead,
it will hold units in reserve and observe

the battle as it plays out. It will then absorb
defeated or weakened attack units back into

the reserves and deploy them to support other
units or outright storm the city when the

If units successfully storm the city, then
a separate sub system will kick in to handle

their behaviour in order capture the settlement.
Now how do each of these five major tactics

In order for the siege system to start selecting
tactics, it needs to know what strategic opportunities

are available to it. As such, each city has
a set of key features exposed by level designers

that the AI can assess in preparation for
combat. It begins by getting a lay of the

land, first by establishing the overall settlement
perimeter and secondly by assigning deployment

lines. The perimeter gives it an idea of where
to attack and assigns deployment lines that

will roughly align with it. This creates lanes
for units to move from deployment line to

city perimeter. The point where the lane meets
the perimeter is known as the attack focus.

The system then assesses each attack focus
for three features:

- The number of dockable wall pieces near
the focus, such that it may scale the perimeter.

- How close the focus is to a gate that can
be attacked.

- How many destructible walls are near the
focus.

Based on the scoring, the Detachment will
assign either a Gate Assault, Wall Assault

or Wall Breach tactic to a unit and assign
that attack focus is their designated target.

One caveat to all of this are the new flying
enemies in Warhammer: these have one simple

tactic and that's attack enemies on the walls
of the city. For everyone else, all tactics

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The Siege Battle AI of Total War: Warhammer (Part 5 of 5) | AI and Games

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