Each time you finish researching a technology in Old World, you are given a small selection of available techs to research next. The semi-random nature of this system means that even if something is available in the technology tree you may not be able to research it right away. This makes it important to prioritize technologies that you need when they are offered.
While every tech in the game has useful applications, the ones listed below are notable for their ubiquity, versatility, or power. Selecting these technologies for your next research ensures that the turns invested are well spent.
10 Aristocracy
This early tech grants unlocks what's likely to be your first Council position, the Ambassador. A skilled Ambassador can improve your relationships at home and abroad, keeping you in the good graces of your rivals. An Ambassador with high Wisdom also contributes to your cities' Culture, letting them develop faster.
Aristocracy also lets you enact either Centralization or Vassalage Law. The former grants extra Civics per turn and boosts the Science output of your capital, while the latter reduces the upkeep of your military units by half. As an added bonus, Aristocracy shuffles the Free Border Growth card into your tech deck, which can vastly expand your holdings if held until later in the game.
9 Drama
Inventing poetry, theatre, and music allows you to construct Odeons in your cities, providing much-needed Culture. The tech also shuffles a Free Settler into your tech deck, helping with early expansion.
The devs also gave Old World a neat flourish with the Drama technology. Until it's researched, you'll only hear ambient sounds while playing. The game's lovely soundtrack begins to play once your culture invents music.
8 Navigation
Sea power isn't necessary in every game, but when it is a factor it's a vital one. Unlocking Biremes with the Navigation technology is essential for projecting power across the waves, holding key straits and transporting your ground troops.
Navigation also unlocks the Colonies and Serfdom Laws, letting you exploit neutral river tiles for extra movement or increase the production of your Farms and Pastures.
7 Barding
Barding is a late-game technology that unlocks the strongest unit in the game, the mighty Cataphract. These mounted monsters have an incredible combat strength of ten and can attack every adjacent unit at once.
Cataphracts are best for empires that have already invested heavily in mounted warfare, but in the late game trying to rush for Barding can even the odds against a superior foe if you can hold out long enough.
6 Forestry
Until Forestry is researched, the only way you can add more Wood to your reserves is to purchase it or chop down scrub and forests. Neither method is sustainable, so you'll have to research Forestry sooner or later if you have any intention of fielding a significant armed force.
Forestry unlocks the Lumbermill improvement which can only be built in forested tiles but provides a steady income of Wood. For best results, place Lumbermills on tiles adjacent to a river to get a sixty percent production bonus.
Forestry also gives you an opportunity to recruit a free Court Scholar later in the game, which isn't strictly necessary but is a nice bonus tacked on to a critical technology.
5 Cartography
Cartography is an important mid-game technology that can either supplement or replace Navigation depending on how heavily you intend to make use of sea power. The main reason to choose Cartography is to unlock the Harbor improvement, which provides a significant income boost to coastal cities and can train idle ships.
Discovering Cartography also lets your Spymaster steal enemy research. There's always a risk of getting caught, but a high-Wisdom Spymaster can significantly hasten your progress along the tech tree with repeated missions.
Lastly, Cartography unlocks the Trireme, an improved warship over the Bireme. It even gives you a chance at a free one by shuffling the Free Trireme card into your tech deck.
4 Stonecutting
If your nation doesn't start with Stonecutting already unlocked, it should always be your first research if available. You're going to need lots of Stone to expand your cities, which means Quarries are a must.
Stonecutting also shuffles a card into your tech deck worth two hundred Stone if researched, which can get you the necessary resources for an early Wonder if timed correctly.
3 Steel Or Phalanx
Typically, you'll want to upgrade your Warriors into either Axemen or Spearmen. You should only take both if you're going all-in on military conquest. Otherwise, either upgrade is fine.
Steel and its Axemen are recommended for fighting in open plains against infantry, as Axemen attack the units to the left and right of their primary target. Phalanx should be taken if your opponent makes heavy use of mounted troops, or if you're going to be fighting in tight quarters. The Spearman's Pierce ability lets them damage the unit behind their target, picking off pesky supporting archers.
All things being equal, you can also consider what each tech leads into; Steel allows you to progress into Battle Line, then from there into Cohorts, unlocking elite infantry units. Phalanx follows into Citizenship and Scholarship, making it ideal for empires pursuing war as a secondary focus. Citizenship is also required for the Stirrups technology, making Phalanx a necessary pick for cavalry-focused civilizations.
2 Monasticism
Once the World Religions (Zoroastrianism, Judaism, Christianity, and Manicheanism) start appearing in your cities, researching Monasticism lets you put their scholars to good use. With this technology unlocked, Disciples can construct Monasteries in cities where their faith has taken root, producing much-needed Science. Monasteries are best built next to Grove improvements, so try to research Land Consolidation soon after Monasticism if you haven't already.
Monasticism also unlocks the Monotheism and Polytheism Laws. Monotheism is excellent if you've committed to a single World Religion and spread it to all your cities, providing an extra Order per turn for each city following your State Religion. Polytheism is great for nations with strong Pagan faiths, as it allows one of each god's Shrine in every city.
1 Lateen Sail
Lateen Sail becomes available once you're researched Coinage. While it does unlock the game's most powerful naval unit, the Dromon, that's not the reason Lateen Sail is a priority pick. Lateen Sail should be chosen because of the exceptional Economic Laws it allows.
Autarky is fantastic for militaristic and builder civilizations, as in increases the output of all Mines, Quarries, and Lumbermills in the empire by twenty percent. Considering how many of each you're likely to have by the time the Law is available, that's a lot of extra resources.
Trade League allows for unmatched resource versatility, letting you sell resources for their buy price. You'll still need to be careful not to drive prices too far up or down, but being able to exchange resources at nearly one-for-one means you can always get what you want when you need it.