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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

  • War: Warhammer.

  • 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

  • find what secrets lie inside!

  • 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

  • the centre of the city.

  • 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

  • brought by the Warhammer IP.

  • 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

  • action to make.

  • 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

  • And lastly - Breaching 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

  • available in the city. and also

  • - 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

  • time is right.

  • 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

  • work in the context of the game itself?

  • 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

  • attached to the focus points is aiming to establish an entry point: a focal point for

  • future storm tactics, where the system switches from destroying the defences to taking the

  • city. In the event the AI is successful in creating an entry point, the settlement perimeter

  • is updated to recognise the new gaps that have been made which is used both by the attacking

  • but also defending AI systems. All entry points are then maintained by a new Entry Points

  • Manager that is attached to the Grand Tactical Analyser in the battle model.

  • So how does each faction execute a given tactic? It's actually kinda variable depending on

  • what resources are available to them - be it the preparation of siege equipment or the

  • make up of their attacking army. Each tactic calculates the estimated time to complete

  • and percentage of completion at a given point, assigns units to a given tactic, with some

  • intended to satisfy the objective and others to defend the objective-focussed units.

  • Each tactic will assign one or more units into an appropriate configuration and then

  • if necessary move into range to conduct the attack. For example, wall assault will begin

  • to move groups with either ladders or siege towers into range and add support units where

  • possible, meanwhile wall breach tactics need to move into firing range, with cannons at

  • front of the formation and catapults at the back. Meanwhile gate destruction tactics are

  • set to have the most destructive and powerful units for that objective at the front, be

  • it battering rams (which require infantry units) or monstrous creatures such as giants.

  • During the assault, high value units such as monsters and heroes are evaluated for their

  • value at that stage in combat, with specific usage flags set by designers that dictate

  • how best to utilise them during a stage of the siege, even if it means keeping them behind

  • and protecting them due to their high value.

  • Each tactic is then executed and observed to see how well it is progressing. Tactics

  • are aimed at being modular but also can communicate with one another: meaning they can allocate

  • units to one another if necessary. For example, when a gate assault tactic reaches the attack

  • focus, it can relinquish units to a wall assault tactic and sometimes vice versa. In the event

  • a tactic still falters, that's where the reserves come in. Any units that were not immediately

  • assigned to a tactic at the start of the siege are assigned to the reserves. In addition,

  • if it becomes clear that a tactic cannot be completed due to heavy defence and unexpected

  • losses, the AI will reallocate the remainder of that unit to the reserves. The reserves

  • then periodically assesses the active tactics to determine their strength and then deploys

  • units in order to support them.

  • But over time the AI hopes to achieve a victory, once a suitable number of entry points are

  • established, the system will execute the Storm tactic, where it forms storming groups of

  • multiple units at specific entry points. In the event that units begin to break into the

  • enemy settlement, that's where the secondary siege systems kick in.

  • Once inside, the settlement subsystem assumes control of any units that have crossed the

  • settlement perimeter. It is then reliant on an established graph of the settlement: dictating

  • how specific areas of the city are linked to one another. In addition, the settlement

  • graph has an influence map overlay attached to it. This influence map carries a number

  • of useful pieces of information at runtime, including the threat posed by a location,

  • the strategic value of capturing or defending it, exposure to missile fire and any enemy

  • occupation strength. This information when summed through the influence map gives a strong

  • indication of whether a given unit could successfully assault and capture a location by building

  • what is effectively multiple layers of useful strategic data into a information model. Think

  • of it like a lasagne for destroying a city.

  • Total War continues to be an extensive and complicated problem space to work within.

  • With each new release we see a need for more complex and robust AI subsystems, from unit

  • movement, to combat, long-term strategy, diplomacy, siege battles and more. At the time of this

  • video, Total War has released not only the sequel to Warhammer, but also announced two

  • new titles: Total War: Three Kingdoms which will return the franchise to the real world

  • with the Han dynasty of China, as well as Total War Saga: Thrones of Britannia - a new

  • subseries focussed on smaller and more specific periods of history. Even from its humble beginnings,

  • Total War was aiming for high stakes and epic combat, but the challenge faced is far from

  • easy. As stated back in part 1 of this series, AI for real time strategy is one of the most

  • complex and demanding out there. They might not get it right all the time, but it can't

  • be denied that Creative Assembly put tremendous time and resource into continually expanding