Defensive Military Operations in Civ-Style Strategy GamesCS 395 GAISpring 2005Problems in Defensive AI Design• How to create a defensible civilization?• How many resources should be devoted to defense?• How should the AI respond to threats and attacks?Defensible CivilizationsDefensible Civilizations• Clusters of cities to create defensive support networks– Distributed production capacity for bolstering a local defensive force– Road networks allow rapid inter-city movement of units– Leveraging terrain features to limit avenues of approach– Well-insulated locations for production centers and valuable assets (wonders)Clustering Cities• Mutual protection– A defensive response force built one unit at a time may arrive too late– Units can defend nearby cities• Exploit a wider range of resources– e.g. specialize in growth vs. production• Prevent a “city rush”Road Networks• Allows rapid redeployment of troops in case invaders arrive• Improves access to resources• Double-edged sword– In FreeCiv, enemy units may also travel your roads– Railroads make this even more dangerous• Zero-cost movement can lead to total collapse in just a few turns• Standing army needed to react quicklyCity Placement• Well-placed cities can be more easily defended• Mountains, water, etc. can limit avenues of approach, especially when undeveloped• Create natural chokepoints– May focus on defending chokepoints instead of individual citiesProtecting Assets• Minimize threat to production centers– Keep them away from the front lines– Use their production capacity, road networks to supply positions at chokepoints• Wonders– Keep them at the center of your empire– Not only very expensive, but draw the attention of enemy playersDefense Resource AllocationDefense Resource Allocation• City garrisons• City improvement trade-offs– Guns vs. Butter • Standing army outside cities• When to shift production to defenders• Demobilizing armies• When to research military techsCity Garrisons• How many troops to leave in your city?– Maintenance cost vs. defense value– Empty cities are easy to pick off, but well-manned cities slow growth• Garrison as policy or in response to threats?City Improvements• Guns vs. Butter– Barracks and other defense improvements can significantly improve defensive position– Cost of maintaining those improvements detracts from improving productivity• Cities might specialize in military vs. research vs. expansion – Determine specialties based on location, available resources, goals, tactical situation– Strengthen a cluster of cities by building a more complete mix of specialtiesStanding Armies• Mobile force for deployment as needed• May serve as a deterrent• Drain on explorationShifting Production to Defenders• When should you make a shift?– Preemptive vs. JIT productionDemobilizing Armies• You’ve just beaten off the invading army–Now what?• Experience army is stronger• But what happens when they come back with tanks?– Demobilization frees up resources for innovationResearching Military Techs• Similar concerns to unit production– Stay ahead or play catch-up when “badderbad-guys” are discovered?Responding to ThreatsResponding to Threats• Standing armies– Distributed vs. Concentrated• Finding the right mix of troops– Land vs. Sea vs. Air• Engaging invaders– “Bait and wait” vs. pre-emptive actionDistributing Armies• Distributed armies– First line of defense for any situation– Tougher to respond with a strong second-wave• Concentrated armies– Good chance of routing invaders on first contact– But risk exposing an Achilles heel • If they slip through to your production center, you’re in big troubleTroop Mix• Land vs. Sea vs. Air– Terrain will be the biggest influence– Weak sea defense can be exploited to bypass strong land defense– Air defense is expensive, but tough to shore up once you fall behindEngaging Invaders• Strike first or wait until they can’t escape?• May depend on your assessment of their forces– If you think you can win handily, you might as well get them too close to escape and then wipe them out– If you’re not so sure, you might want to strike hard early and hope they think twice about their plansFlexible Planning for DefensePlanning Problems• Global vs. local planning• Pursuing multiple goals in parallel• Making plans that support a fall-back option – Inexpensive/partial replanning– Knowing when a plan has gone awry• Reactionary capabilities• Opportunity costs of plans/actionsSearch Problems• Intelligence on other players– Based on observations, what might they have?• Finding chokepoints to defend• Pathfinding• Optimizing unit/tech choices based on current events• Wonder production based on what others have been producing• Cashflow