The Fezzan Corridors
- is Fezzan.
This is probably one of the hardest achievements in the game, and requires a good understanding of estates and how they work.
The trade nodes need to be controlled by over 90%. To get that high, there can be no one else owning the downward trade nodes since they can drag power from the four required nodes. So conquering a large part of the Sevilla trade node is necessary to be able to get to 90+%.
The game starts the player out between a rock and a hard place. Tunis and Mamluks will probably want to make Fezzan their vassal, it is important not to accept it. The best way to prevent a failure is an early and quick expansion. The start is very tricky: Before Tunis warns the player, it's possible to fabricate claims and go to war with neighboring minors, a mission might be available to attack Djerid, but if not, rival them and, if that doesn't help either, use a no CB war so you can do it before before Tunis sends warning, the player should use the war to either make Djerid a vassal or conquer it, but the warning will come very early in the game and limiting the options significantly.
Allying all other nations that accept it: Morocco, Tlemcen, Touggourt and Mamluks might help. Use them to attack Tunis and conquer the provinces needed to form Tripoli, and maybe some more. Forming Tripoli will upgrade the player into kingdom rank and makes Tunisian the primary culture.
After that, clean up the minors using conquest, integrate Djerid, and get ready for a second war with Tunis. During this time it is good to improve relations with the mighty Ottomans, to gain an ally against Mamluks and the Iberians. However, this can be quite difficult before forming Tunis. It should be noted that forming Tunis will also give all former Tunisian cores to you as well (since you are now Tunis) while the requirements to form Tunis only require 3 provinces to be cored and for Tunis to not exist. It might therefore be possible to get some free cores out of this decision
The player should use the allies to take out Tunis, and then reform as Tunis. This should make it easier to please the Ottomans and to secure an alliance. If this doesn't work: Either rival the Mamluks or get a royal marriage to make the Ottomans friendly towards the country
After forming Tunis, the player should seek to dominate North Africa, perhaps using Morocco against Tlemcen, and conquering Morocco soon after.
When the Ottomans go to war with Mamluks, join in and take a couple of provinces. The player is going to need all coastal provinces they can get to gain naval superiority, because the Iberians will not be friendly. Having a lot of ships to stop them from dominating the strait of Gibraltar will be important. Building shipyards is expensive, but necessary.
The player's income could be struggling at this point. Moving the main trade city, privateering and raiding can help with finances. Just make sure to have a good combination of warships and light ships, because not only because gold is needed, the player also needs a good war fleet to subjugate the Iberians.
It is recommended to take Exploration as the first idea group so that the player can fill in the gaps to the south and get some income and start conquering south of the Sahara. There are some gold mines there that will help supplant income. Taking gold and war reparations as peace terms will also help keep the player's finances afloat. Other recommended idea groups include Maritime, Trade, Economic (inflation reduction will help with the gold mines the player might acquire), and Quality (helps with naval warfare as well as land).
When the league wars (or any war between France and Castile/Spain) happens, take help from the Ottomans and conquer some Iberian provinces. Aim for the provinces needed to form Andalusia and those that give trade power in the Sevilla node. Forming Andalusia allows a free move of capital to Europe and the Sevilla trade node. Owning it is essential to get the achievement.
By the end of the mid-game, the player should seek to own a large part of the provinces in the four required trade nodes, along with part of the Sevilla trade node and preferably part the upstream trade nodes in Kongo and Cape.
Nearing the end of the game, the player should strive to own all provinces in the four required trade nodes. It is also preferable for the player to control the provinces in the upstream nodes of Kongo and Cape. Additionally, controlling a large part of the Sevilla trade node will prevent trade power from leaking into it from the four required trade nodes.
Continuing to increase trade power by building trade buildings and developing provinces should help the player reach 90% trade share in each node, completing the achievement.