The Late-Game Strategy
The late game is generally considered any turn where most (if not all) of the players have progressed to the 7th level of the technology tree (e.g. Metallurgy, Steam Engine, Explosives. and Leadership). This is where things can get really interesting.
Diplomacy
At this juncture of the game, if you have not determined who your friends are and who your enemies are, then you are in trouble. While there is no requirement to formally declare an in-game alliance, you should have already started discussions with other players about combined or shared plans. Most players that survive this long are good players and will also have developed alliance plans.
Note
Every Longturn game has different alliance victory rules and conditions, so make sure you are aligned with those rules before getting carried away.
City and Terrain Management
If your form of government is not Diplomacy or Federation and you do not have the mass majority of your cities celebrating, then you are behind. By the time you have learned all of the Level 6 technologies you should have many cities already celebrating. The Gold and Luxury output will greatly boost your gold per turn (GPT) and bulbs per turn (BPT) capacities. You can rush-buy buildings at this stage if you planned things right. Gold ends up being your production “super power” at this stage of the game.
Keep those Workers going. Get a Sewer System in your cities so they can grow bigger than size 16. Once you learn Explosives, you can either use gold to upgrade the Workers to Engineers or allow Leonardo’s Workshop to do it for you. Every tile should be grassland by now (or at least the majority of them). Use Engineers to cut down Mountains and Hills (ones without a special on them). Use the Engineers to get rid of Desert tiles (ones without a special on them). This is the time of the game where the majority of your cities should be at \(+20\) food and growing every turn.
This is also a time in the game where pollution can cause Global Warming. In LTT and LTX games, Global Warming is very powerful and can wreak havoc on your growth plans. Keep tiles clear of pollution as best you can.
Tip
Transports can clean pollution from shallow and deep ocean tiles. Best to put them with a defender as they are sitting ducks to your enemies.
Note
Do not be surprised if you see other players ignoring their pollution. This is a known strategy to force Global Warming on everyone. You can certainly attempt to get them to clean up, but do not be surprised if they ignore you.
Tip
With Engineers, you can turn a city center tile from any type of terrain into Hills. The terraforming exercise is very expensive in worker points and will take many turns, even with a large number of Engineers working on the same task. However, the defense bonus will make your cities harder to capture and also will not be impacted by Global Warming’s effects as badly. It is advised to do this for your capital at least.
Research
What to research at this point in the game can vary widely depending on what your strategy is with your alliance. Different players in an alliance can research different paths to specialize. For example, one player can go for all the sea-going technologies (Steam Engine, Railroad, Engineering, Steel, Combustion, etc) and develop a very strong Navy. Another player can concentrate on air-based technologies (Flight, Radio, Advanced Flight, Robotics, Combined Arms, etc) to build out an Air Force and a third player can concentrate on a traditional land-based mechanized Army (Leadership, Tactics, Amphibious Warfare, Labor Union, Robotics, etc.). When it comes time to defeat your enemies you can work together to bring to force what is needed at the right time. There is no set prescription this late in the game, you will have to use your best judgment.