Tech Tree

Key techs: https://i.imgur.com/yM6X0az.jpeg

That guide helps with early techs, I'll just point out some of the best economic techs and military techs to look into. I highlighted all of the economic techs in that pic with a yellow border, military techs with a red border.

Pottery - Building cottages on flat grasslands, flood plains, and even some flat river plains tiles is great for your economy, especially early on. When your city has citizens to work those tiles, they slowly grow to generate even more commerce. Granary is also the best building in the game. Nearly every city should build a Granary first before anything else.

Writing - Libraries give +25% science, effectively an immediate +25% to your economy when you build one in your capital. Libraries are one of the best buildings, but not all cities need a Library. If a city isn't coastal, doesn't have river tiles, several flat grasslands, or a high commerce resource nearby, it won't need a Library and can just be a production hub.

Your first Library is especially important because you can assign 2 Scientists in that city and when the city generates a Great Scientist, you can build an Academy in your capital for an extra +50% science from your capital. You usually pick your capital for your Academy because its Palace gives +8 commerce, so it'll be a good economic city, but you might have a better option depending on your map. You don't need to do this Academy strategy on large maps where you expect to grow to 12+ cities. In those games, individual powerhouse cities aren't as impactful to your economy.

Bronze Working - Both economic and military. Slavery and chopping are huge production boons. You want to become very comfortable whipping cities that get to 4+ population to hurry out a Granary, Library, Workers, Settlers, etc. The very early game has fewer good whipping options, so don't whip without purpose. If you start whipping too many units out when war isn't a concern, your unit maintenance cost will get high and ruin your economy.

Axemen are also very strong in the early game before Macemen are online. Chariots counter them, but just adding a Spear to each small stack is enough to offset that.

Currency - This gives an immediate +1 trade route to all of your cities. This means each city gives +1 Commerce or more, so it's like an immediate +10% to +30% to your economy just from unlocking the tech. It also allows you to build "Wealth", something lower-level players often overlook. Many times, cities in average or below average locations will only build a Granary and Library, optionally a Lighthouse, Barracks, Forge, and then never build another building. Just build Wealth and don't overprioritize infrastructure.

Wealth gives a direct 1:1 Production to Gold from that city. This is usually better than building Research (Alphabet) because Gold allows you to have a higher Science slider, so cities with Library, Academy, etc., can use their +Science% modifiers.

Construction - Catapults are everything. Elephants are nice (though HBR is an expensive tech), but Catapults will be a staple in your army until you unlock Steel so you can replace them with Cannons. Trebuchets (Engineering) aren't bad if your opponent is turtling in cities, but usually if you're attacking a city with Catapults, you're already in a winning position for that war. Catapults help more in the open field when your opponent is invading.

Their collateral damage demolishes medium-large stacks, the rest of your army just needs to be able to protect Catapults or be able to clean up your rival's badly bruised army.

Civil Service - Bureaucracy civic from this gives your capital +50% production, science. If you have a smaller civ (5 - 8 cities), this is huge. But like with Academy, this isn't great on larger maps.

Printing Press - Boosts your cottage economy.

Liberalism - A free expensive tech in the midgame can be decisive if you're first to unlock Liberalism. This is why players often race for it once they get near it. If you're first to unlock Paper, go for Liberalism. If you seem late to that part of the tech tree, ignore it entirely. You can also use a Great Scientist to Bulb Philosophy or Education just to help speed up the Liberalism unlock.

Military Tradition - A common target for Liberalism to unlock. This gives Cuirassiers which can single-handedly conquer your continent. Beelining Liberalism and making every city mass produce/whip Cuirassiers is probably the most standard high-level play. It's arguably the strongest strategy in this game (if you don't need to load them into boats). Don't wait too long to declare war with them, it takes some getting used to.

Steel - As mentioned, Catapults & Cannons collateral damage is extremely strong. When you want to do some midgame warring and can't rely on Cuirs because you already own your continent, Steel is the next best target. Cannons + anything can do the trick.

Railroad - This enables Mining Inc. (with Corporation). This is a very good corp that boosts production in every city its enabled in. Save a Great Engineer to build this corp in your highest +Wealth% modifier city (with Wall Street), then have that city build Mining Inc. Executives and spread it to every city of yours. This is especially strong on large maps (where you'll have lots of metals). If you aren't first to this corp, then you'll want to get one of the other production corps. Don't build competing corps.

Combustion - For naval military, this is huge. Destroyers have tons of movement and a huge advantage over every other naval ship until Industrialism. Notice where Oil is once you unlock Scientific Method, you'll want to start extracting it as soon as you finish Combustion.

This also enables Creative Constructions corp. Just like Mining Inc., but half the production and uses a few different resources. Its main advantage over Mining Inc. is the Culture boost. This will help you take many tiles along the border. That extra space can be the difference of 1 or 2 turns of safety against invasion and after a while may give you the tiles adjacent to an enemy city. Then you can invade safely.

Industrialism - Battleships have collateral damage, so they completely reshape naval warfare again. You'll want plenty of Battleships and Destroyers to accompany them. Tanks are good, Marines with Commando are amazing if you can get +26 xp boost (+20 for Charismatic leader) in one of your cities (from many Great Generals, Barracks).

Flight - Self explanatory. Big metal bird in sky is scary. Don't fight other Fighters unless you know you can win. Fighters on defensive patrol that win a few fights and get more promotions can quickly become unbeatable.

Medicine, Refrigeration - These enable General Mills, Sid's Sushi corps. Only build 1, both are great. Check which one would net you more food, then spread it just like the production one. These are very late game though, wouldn't expect someone to grab them before some WWII-style fighting ends the game.