My starting game was similar to the suggestion above:
Day 1: DOW Albana and send the barque to get 25% war score from blocades. With the other diplomat, send warning against the Ottomans. Have +5 Dip Rep advisor, otherwise restart (you need it for fast marriages/alliances). Start mission to accumulate money or repair relations with Athens. Build a Cavalry and Infantry, make your king a general and start transporting your units to Albania. Afterwich, improve relations with Poland, guarantee and if possible make rival their rivals. The other diplomat can be used to improve relations with Lithuania/Austria/Greece.
Rivals early game are usually:
Poland: Teutonic order
Lithuania: Russia/Muscovy and Ottomans!
Austria: Burgundy
Muscovy: Novgorod
After you landed your 8 troops in albania, declare war against Serbia and send them directly to the Serbia capital to crush their low morale army. When you DOW against Serbia you should have 95-100 Admin points to reduce by 1 the stability cost. After your troops are in Serbian territory, you can offer peace with Albania: Vassalize + cash (usually about 33 ducats if your war score is at 23%). Then the next day pay cash the Ottomans for peace. They will warn you, but its of no consequence since you already declared war with Serbia
.
After you crushed the Serbian army, carpet siege all provinces even if you can't siege them. During that time try to get a marriage with poland and continue improve relations. Don't ally/marry anyone else.
After taking 3 provinces of Serbia (~83% war score), take Zeta as your own province (so you can fabricate claims on Bosnia), vassalize Serbia and ask for cash. At peace, make an alliance with poland and if you worked the relations high enough, press claim throne during their interregrum. usually by december 1447, a new ruler will be chosen in Poland and if you are lucky, they will have the same dynasty as yours and no PU with Lithuania
. Afterwhich you can start marry/ally Lithuania and another big country like Austria if you fear Venice.
After your claim is done, DOW Bosnia, and if they are allied with Wallachia, vassalize both of them. In the meantime, their is a good chance that Cyprus must of been conquered by the mamluks and rebels will spawn (16 stack) and flip to you. Other islands might flip too without the need to support rebels.
You need to have strong allies before 1450 so the Ottomans won't dare to attack you, and you will most likely have conflicts against Venice than others. Start integrating your vassals Athens and Albania when you can so you can start earning more diplo points.
After that, the next wars can be Trebizond (you have core there), Georgia if they are eaten by other can be a great vassal for feeding :x, Tunisia if they have less allies than at game start, I also released croatia early on to grab some land from a weak hungary, and if Naples are not allied to Aragon or under PU, then start your Italian conquest as early as possible.
Never attack a HRE country and you should be fine. Never refuse a call to arms so your trust remains high and Austria or Russia won't break their alliance. If Austria is weak and France is strong, go with France. I tried an alliance with a strong Castile and this is what happened:
Castile does the reconquesta and drags the ottomans in the war. The ottomans destroy me, the spanish troops are not even in North Africa and just sit there in the Iberian peninsula. Meanwhile the ottomans siege all my land, then "Game over". So I don't recommend Castile as an ally until much later.
After you beaten down the European part of the Ottomans, don't continue, try to make other Muslim countries weaker such as the Mamluks. When the Muslims in general are in shambles, then you try to go for the rest of Anatolia, Jerusalem, Alexandria and Tunisia.
I tried this start about 20 times, they usually work pretty well but I made blunders later on that forced me to restart (like the Castile example)
.
Another Tip: don't try to attack the Ottomans too early. You can have a great start and quickly ruin your game. Make sure you are ahead of their military tech, because the Ottoman tech is very strong the first 50-100 years.
Also you need to continuously dominate the seas, even if they go around the black sea, this will give you and your allies to build a powerful stack. The first 50 years, the Ottomans will usually have 12-15 galleys and 4-8 barques, so you will need more galleys. Their maintenance doesn't cost much and I find it more important to go over the naval force limits than the land force limits.
Make sure you go to war with Allies that are ready to support you. I found Lithuania very effective (much more than poland), meanwhile Austria can be a good ally unless its at war with Burgundy or France (The Burgundy conquest of Liege can destroy Austria and force them to release Tyrol). 50 years forward, if Russia is strong enough, they will become your best ally because they are by far the strongest AI nation in the game after a while, and the best part, they are Orthodox. Of course this will make Lithuania angry, but if they are in a PU with Poland, the latter will eventually break off the alliance because of their interest in Constantinople after integrating Modalvia.
Venice is always a splinter in your plans, they love to DOW on you the moment you are a bit weak or if you are at war. Having Austria as an Ally will help to crush them quickly, meanwhile your eastern allies will just stupidly stack in your lands and suffer attrition. Honestly the allies are so broken when playing the Byzantium.... Early game, I find it hard to find a good ally. Hungary in all of my games have been useless. I prefer to kill Venice before I kill the Ottomans, because they are so annoying and a huge naval fleet!
The Ottomans can always be a threat and I have seen them in an Alliance or even leading a PU with Qara Qonlyu (however you write it...) and that can be an extremely powerful alliance.
Hope that the Mamluks become hostile against the Ottomans, and when the Ottomans is under 15-20k armies and almost no manpower, then you can attack (make sure their allies are weak too). If the Ottomans suffer war exhaustion from other wars, they are most likely to settle peace quickly with you = More lands gained, less deaths
.
