- This article is about the expansion. For the other uses see 我们的海（消歧义）.
- Barbary nations can raid for slaves with their ships.
- Steal Maps is a new spy action that lets you conduct a despicable act of thievery and discover all provinces your victim already has discovered in a region.
- Hunting naval mission: Fleets will hunt enemy fleets in a specific region.
- Evasion naval mission: Fleets will evade enemy fleets in a specific region.
- Blockade naval mission: Fleets will blockade enemy ports in a specific region.
- Intercept naval mission: Fleets will seek out and attack enemy fleets near allied coastlines, focusing on troop transports.
- The new diplomatic option Unconditional Surrender forces your enemy to send a peace demand for up to 100 WS within two months, or Call to Peace will be gained at twice the normal rate.
- Having a spy network in another country now gives you passive benefits to siege ability and aggressive expansion incurred against that country.
- Three new espionage actions: Slander Merchants reduces another country's trade power for 5 years, Sabotage Recruitment reduces manpower and sailor recovery speed for 5 years and Corrupt Officials increases corruption.
- The Mare Nostrum DLC also unlocks the Hunt Pirates mission (from the El Dorado DLC).
- Unit View: Added Naval Mission Settings panel that allows you to adjust the behavior of a fleet when on a mission.
- You can now rent out Condottieri to other countries.
- You now press sailors automatically when occupying coastal provinces.
- "Detach Damaged" is changed to "Repair Damaged". Damaged ships will head to port for repairs and then reunite with the fleet they were detached from.
- You can now send a diplomat to conduct counterespionage against a country, increasing the chance of detecting their attempts to build spy networks by 100%.
- Naval leaders are now allowed to be reassigned to and from ships within supply range.
- Promote Mercantilism: Spend diplomatic power to strengthen your mercantilism.
- Added a Timeline feature that allows players to observe how the world has changed throughout the game.
- A leader of a Trade League, can now create a Trading City from one of its provinces. This new nation will have the Trading City republic government and will automatically join the Trade League. (introduced with patch 1.18)
- Majorly reworked Espionage. Instead of sending diplomats to carry out sustained spy actions, there is now a new 'Create Spy Network' sustained action, where your diplomat covertly builds up a spy network in another country. Spy networks are size 0-100, and espionage actions other than Build Spy Network are now instant actions that cost a certain amount of spy network points when used but are guaranteed to succeed (for example, Sow Discontent applies the Discontent Sown modifier to the target country for 5 years). Spy Offense modifier was changed to Spy Network Construction and determines the speed at which networks are built, while Spy Defense determines the chance of detecting when another country is building a spy network in your country and disrupting their efforts.
- You can now grant a Vassal their Core Province even when it isn't adjacent to them.
- Added a Loot Mapmode.
- Study Technology is no longer an action but a discount to technology cost gained from having a spy network in another country with more advanced technology. Size of spy network and how many techs they are ahead of you determines the size of the discount, with only the largest discount for each category used (so if you get 20% land tech discount from one country and 15% from another, your actual discount is 20%).
- Naval Combat: There is now a chance of capturing ships that would otherwise be sunk, based on Admiral Maneuver skill. (This is in addition to ships with low morale captured after the battle.)
- Naval Combat: Losing a ship now causes the entire fleet to lose some Morale.
- Naval Combat: The Positioning concept is removed.
- Naval Combat: Introduced "Engagement Width", which is the number of ships that can engage the enemy each turn. This is modified by leader maneuver skill and local/global modifiers.
- New mapmode: States and Territories.
- Added the historical_neutral tag which makes countries less likely to become friends or enemies.
- Colonists in Trade Company provinces do no longer change the religion or culture of the province.
- Admirals now have siege ability which increases blockade efficiency by 10% per point.
- Merchant marine now gives +50% Sailors & Press Gangs now give +20% Sailor Recovery.
- Added mechanics for Sailors, given from development in ports. Sailors are used when building ships and repairing them.
- Added continue button to end game screen to allow you to go beyond the supported end date if not playing ironman.
- The degree to which a trade node is monopolized (as determined by the greatest power share) now adds up to 100% to the privateer power modifier.
- Province tooltips now show more information regarding blockading fleets.
- Blockades are now indicated on the map with special border graphics.
- Implemented a new system called "States & Territories", where states gives most benefits of being non-overseas, while territories have autonomy and is considered to be overseas for many rules.
- Added Corruption, which is impacted by being behind in tech, being a big nation, and can be combated by investment in the budget. Corruption impact minimum autonomy in a country, its ability to do espionage and all power costs.
- Added "corrupt" cheat to set the level of corruption.
A list of the dev diaries for Mare Nostrum can be found here.
- Forum, The next Europa Universalis IV expansion: Mare Nostrum!, 01-03-2016
- Forum, Patch 1.16 * FULL patchnotes, 04-04-2016