Technical and performance improvements:

Performance and memory optimisations in Campaign modes.
Further work to reduce the CPU costs during battles.
Improved framerates when units clash in battles.
Siege battle performance optimisations.
Eliminated several rare crashes found in battle scenarios.
Fix for rare crash related to naval boarding/disembarking
Multiplayer battle-replay desync addressed.
Issues with trees in battle-maps addressed.
Additional cost-balancing to Chariots and Elephants.
Units no longer mistake bastions for wall breaches in settlement battles.
Further battle map technical and performance improvements.
Support for AMD switchable graphics:

Game now correctly detects and reports the discrete GPU on an AMD switchable graphics configuration (primarily laptops with an Intel integrated GPU and a discrete AMD mobility GPU).
This fix ensures that the ‘unlimited video memory’ graphics option is enabled correctly on AMD switchable configs.
Note that the Catalyst Control Centre Switchable Graphics UI must be used to select between the discrete and integrated GPUs for each application that the player wants to run.
Support for ‘Virtu MVP’ Desktop Switchable Graphics:

Virtu MVP is a common OEM-installed application for selecting GPUs in a multi-GPU desktop configuration e.g. a desktop with an Intel integrated GPU and a discrete desktop GPU – both typically routed to the same motherboard display output connector.
We now correctly detect and report the correct GPU used in this scenario.
Note that the Virtu MVP Control Panel application must be used to select between the discrete and integrated GPUs for each application that the player wants to run.
New option in the Advanced Graphics Settings menu, for selecting between graphics cards / GPUs when more than 1 is available.
Support for multi-GPUs with multi-monitor configurations:

Game now detects multiple GPUs and allows the player to pick which GPU is used to render the game.
In this configuration multiple GPUs are shown in a combo-box in the Advanced Graphics Options UI.
Improved video-memory detection to prevent the game from sometimes detecting the video memory limit from the wrong/lower graphics card in a multi-GPU configuration.
Battle AI and behavioural improvements:

Improved pathfinding of siege vehicles near walls.
Improved multiple siege ladder functionality parameters.
Improved siege vehicle docking placement.
Units now consistently disembark from ships with raised bows.
Improved AI infantry awareness of cavalry, making infantry brace when threatened by a charge.
AI controlled units are now more likely to use flaming arrows against elephants where available.
Improved logic of multiple battle maps, to allow the AI to interact with these maps better and cause fewer behavioural issues.
Improved AI General's use of special abilities.
Rebalanced AI's battle-plan analyser/attack-or-defend decision making, to take into account that the enemy alliance may have weaker but longer-range missile units, to prevent static behaviour under missile fire.
In ambush battles, the defender’s units which are under missile fire that outranges their own are now more likely to respond by moving to intercept the missile unit attacking them.
Altered some of the AI's usage of various ammo types.
Pike Phalanx now reform properly once engaged in melee.
Idle units attacked when set to Formed Attack attempt to keep their current facing.
Formed attack charges will now penetrate less than free attack barbarian charges.
Attacks from units in formation are now more powerful than attacks from unformed units.
Reduced the chance of units walking while routing from the battlefield.
Routing units no longer turn around/play matched combat animations as they are being killed.
Units now no longer head through breach in a wall rather than using the siege engine they are attached to.
Artillery with special ammo now show the correct model while loading the weapon: e.g. animal carcasses.
Reduced instances of unit collision which caused jittering on the battlefield.
Eliminated a battle replay desync caused by presence of war dogs units.
Units now correctly stop firing at a building if they take ownership of it.
During multiplayer siege battles, the defender can now see the attacker's units during deployment.
Rebalanced hit-point bonuses for officers and standard bearers.
During battles, units can now burn down open gates with torches.
Men positioned on a gatehouse will no longer die when the gate is destroyed.
Artillery ships can now consistently disembark.
Added more effective collision detection to barbarian watch towers in encampment battles.
Charging at Pikemen now consistently selects the correct matched combat animation, keeping pikemen in line more effectively.
When dog handlers unleash their dogs, the handlers will no longer attack as well.
Added blood to chariot horses and drivers, and to attackers and defenders during knockdowns. (Only active if the player owns the Blood and Gore DLC.)
Artillery on ships can no longer be picked up (previously resulted in loss of unit control).
Improved unit reforming on the battlefield.
Projectiles no longer remain in mid-air when the siege weapon that fired them is destroyed.
Improvements to naval disembarking functionality.
Technical improvements across a range of battle maps (including pathfinding, deployment, general unit interaction, walls, gates, battle tooltips etc).
Multiple minor unit behavioural improvements.
Campaign AI improvements:

AI no longer underestimates its strength when ending a move in Forced March stance within a settlement.
Campaign AI now considers maintaining sieges for longer, in order to build more siege equipment.
Campaign AI now less likely to sue for peace shortly after declaring a war.
AI factions now recruit more siege units.
Revised the Campaign AI settlement occupation decision system.
Adjustments to Campaign AI Financial Management.
AI factions are now much less likely to declare war on distant factions.
Adjustments to composition of recruited forces.
Improvement to Campaign AI food management.
General Battle Improvements:

Eliminated an exploit allowing players to bypass unit-caps in custom and multiplayer battles by modifying a saved army setup.
Units can no longer use loose formation when manning siege equipment.
Buhen (Egyptian battle map) is now playable in custom battle mode.
Caltrops are now placed in more uniform rows.
Tortoise formation is now unlocked in the Seleucid faction when the player owns the Greek States DLC.
Improved multiplayer responsiveness: movement paths and attack arrows are now shown instantly upon giving the order. (Previously the game waited for all clients to confirm the issued order, causing a small delay).
Multiple combat animation blending tweaks.
Orders issued on lower framerate machines now register more effectively.
Added a horse bonus upgrade icon to show different levels of horses on their unit cards.
Improvements to group collision sounds during battles.
Pre-battle voiceover no longer occasionally cuts out.
Tweaks to some Hellenic unit looks to make them more distinct.
Missing rock/tree textures on battlefields added.
Many more siege battle map improvements (e.g. improving docking point placement on walls, joining small gaps in walls to improve pathfinding, removed obstructions from docking points).
General Campaign Improvements:

Added a countdown timer to the Exchange Panel in Multiplayer Campaign mode.
Settlements on the campaign map will no longer continue to emit smoke from damage or construction when they are no longer damaged/under construction.
Right-clicking the mouse during the end-turn cycle in campaign while an agent/army is selected will no longer trigger audio responses from the selected agent/army.
Opening and closing the pause menu, while the Declare War On *Overlord/Make Peace With Satrapies panel is open in Campaign mode, will no longer close the panel and make peace with the satrapies.
Help I'm trapped in these patch notes.
An army in Forced March stance will now have their Recruit Units button reactivate immediately when they are put back into the Default stance.
Replacing an immortal general will no longer trigger a General Wounded message in Campaign modes.
The Zoom To Location button no longer pans the camera to the bottom-left corner of the Multiplayer Campaign map when pressed with an army selected.
The Nervii faction is now immune to snow attrition in the Grand Campaign.
If a general reaches level 25 in a statistic, all of the attribute increases and abilities granted by that statistic will consistently remain.
Addressed a rare issue during campaign ambush battles where, if the user conceded defeat, they might suffer no losses and could then retreat.
When recruiting a unit on the same turn as a researched technology that replaces that unit type is completed, the unit recruitment will no longer be cancelled.
Fix to allow Praetorians to be upgraded to Praetorian Guards once the appropriate technology has been researched.
Fix for Athenian and Egyptian naval recruitment experience bonus in campaign modes.
Battle rebalancing:

Improved behaviour and benefits of Formed Attack ability.
Slightly reduced the spacing of melee units.
Widened the pike cone-of-engagement, so they can hold off units more effectively.
Reduced pike damage.
Reduced the mass (and therefore impact) of chariots and elephants.
Major adjustments to unit costs, to better reflect the combat value of units; there is a much greater spread of values now. For example Oathsworn now cost 1340, Celtic Warriors 350.
Adjustments to unit speeds, stats, and more to help diversify the unit rosters, and give more distinct roles on the battlefield.
Improved AI army compositions in campaign mode, when a faction doesn’t have access to early land or navy units.
Campaign AI is now more focused on acquiring early military technologies.
Certain total casualty morale penalties have been set to 0 so there is less stacking of morale effects.
Reduced minimum hit chance, base hit chance and maximum hit chance for melee combat.
Further balancing changes based on Patch 9 Public BETA feedback:
-Reduction in melee attack for Roman melee infantry units.
-Adjusted costs of missile infantry.
-Adjustments to melee cavalry weapon types.
-Tweaks to shock cavalry costs.
-Added passive “Scare” ability to Night Hunters.
-Increased Wodanaz Spears melee attack.
-Reduced cost of elite Germanic and Briton units.

Auto-resolver balancing:

Smaller defensive bonus in minor settlement battles
A small global penalty for the player in autoresolved battles
Unit threshold has been increased, so severely damaged units are less likely to survive an auto-resolved battle.