Interested in getting the Kingdom of David achievement (As a Jew, create the Kingdom of Israel), I had a few run a Semien (some duchy in Ethopia).
It was not a success: playing non-aggressive and slow expansion let you weak and, since you are of another religion, you get chomped quite fast as the easiest to grab territory in holy war; playing agressive allows to grow very fast but soon enough you are way too threatening and get attacked by coalitions of both opposing religions in the area (miaphysite and islam).
It made me, though, realize that Egypt could actually be a nice starting location for an orthodox game, since Tulunids muslim have opporunity to switch to christianity and having ethopia, if not allied, as safe border. That would make Pentarch (as an Orthodox Christian, hold Rome, Constantinople, Alexandria, Antioch and Jerusalem) and Protector of the Holy Places (have Rome, Jerusalem, Mecca and Medina within your Realm) doable at once.
1. Christening Egypt
First step is to convert to christianity to be able to expand in Arabia through holy wars. That requires an united country, so I revoked my starting sultan’s sons duchies and, at the same time, managed to get my heir educated by some miaphysite character. In just on generation, the switch was over and most provinces were converted too.
The annoying thing, though, was that the change of religion also change inheritance law. As muslim, you start with said Open succession law: the most powerful descendant inherits. Since it is tied to muslim rulers specifics (cons and pros), I understand this kind of succession law is not kept to avoid otherwise gamey conversions. Still, once christian, I got Gavelking succession, which is strange, since it is quite unrelated to the previous one.
So the next urging step was also to change to another type of inheritance: seniority was readily available so I went for it.
2. War on islam
Next, I used holy war to expand in muslim territory. The fact that Egypt starts with cheap half-mercs Mamluks was a plus.
Being christian allowed also numerous wedding with orthodox. It made the switch to an orthodox ruler quite straight forward.
Along with a policy of creating arch-bishops vassal instead of dukes, the territory get consistantly converted.
Since it is all based on holy wars, where any previous ruler is removed, it is quite convenient to shape the kingdom so no vassal got a massive united block, beside yourself.
Aside from all my arch-bishops, I created some Republic town vassal, in order to have one vassal I could give many duchies (impossible to give more than one to bishops, you have to wait for them to usurp a title to get several; too annoying with a real duke that could, in a matter of generations, sprout into splitted multiples dukes all annoying to deal with).
This Doge was never really satisfied, always joining as soon as you can whatever faction (you name it: claimant support, change of succession law, more power to the council, etc) so it had to be kept as small as possible, giving him duchies with none of the lands it relates. I also almost always got my king to be focused on intrigue in order to get this convenient “threaten … to stay out of factions” action on my vassals. It works quite well, either he accept or you have valid cause to imprison him and revoke title. If one faction is actually threatening, it is best to cut it to pieces by imprisoning/fighting one of the main characters than waiting for the whole faction to grow big and pose its ultimatum.
Aside from that, I create a mercenary company and dissolved it. It does not provides that much cash but remove too much of your own levies – which is really annoying since it makes small factions feels bigger, since they evaluate their power by comparing levies size.
Soon enough, I was powerful enough to handle whatever all the united muslim sultans could throw. The extra > 10000 mamluk troops was a great bonus, plus it misled the computer when assessing my troops at the ready. Obviously, I kept lot of cash in treasury at all times, for the start of war when some additionnal expensive mercs were required.
So getting Mecca/Medina would not be a problem from now on.
3. War on christians
Next step was to get both Constantinople and Rome. It seemed to me I could easily get Elected Basileus of Byzantine Empire so I decided to swear fealty and join it. Also, Byzantines had foothold in Italy, so that could be helpful to get Rome.
Rome was still independant so I managed to get a claim and submit it. Byzantine was not an electived title at that time so I was not getting any of it. Worse, I lost a bit of territory due to some mismanaged inherintance. For instance, one of my vassals managed to get the title of King of Africa (titular title) and so was then just under the Basileus, having a title of the same rank as me (that should not happen if Crusader Kings II had a finer understanding of vassalic links: my vassal for duchy X could also be titular king of Z and vassal to the Basileus for that. Gaining an unrelated title does not give you free reign to everything).
For a while, I enjoyed belonging to this big empire so no catholic in Italy would challenge my belongings there, including Rome. But these inheritance issues made be go to war for independance as soon as possible – as soon as there was a big revolt in the Empire.
I had to deal with my sharing of revolts but where all manageable. Only twice I had to accept factions request (empowered council – that I shut off as soon as possible next by buying favors) because I was in the middle of another major war.
4. Expanding in Europe through weddings
Seniority succession makes it actually very easy to expand through weddings. Males easily get decent offers to mary female potential heirs (when they are not directly heirs on the day of the bethrotal). You can force heir with bad traits to go nun/monk.
Soon enough, I found many heirs that we’re already duke of something somewhere. Some died before inheritance but I managed to get several duchies or kingdom as far as Lithuania or Mercian or in the middle of Byzantium Empire.
Seniority inheritance means your average heir wont be living long. But you get no regencies and it no longer really matter whether the heir as overal good traits or not, since they are short lived. It matters only that they bring more titles within the realm.
One drawback, is that you change culture often: so any culture-related building on your holding get lost (if anything, you should loose the benefit of the building an allow you to pay to convert it, but not make it disappear in thin air). Also, you change sometimes of christian religion (catholic, etc), that gives you an annoying malus towards vassals, prevent you to renew your court chaplain to proper orthodox if you are not.
There is a fix for that: later in the game, when I got Empire of Arabia, in intrigue, it allowed me to switch ruler to egyptian culture and to orthodox religion. It was not the case earlier, not with custom Egypt Empire for instance. I do not know if it is intended like this and why it would be, if so.
5. Building number one world Empire
At some point, I had multiple kingdom titles and was the biggest real on all accounts (gold, size, army). And enough prestige to create an Egyptian empire: that is not so easy with seniority and short lived rulers. If your ruler was not duke of sort before inheritance of the main title, it is unlikely he will reach the required 8000 prestige.
Getting Constantinople now was just a matter of striking at the best time. Problem is, all this expansion rose dangerously the threat level.
6. War on indians
At the same time, I noticed I could easily grab an indian kingdom, getting both Saint Thomas’s Dream (rule an Indian Kingdom or Empire as a Christian and convert all its provinces) and Beyond the Indus (as a ruler of non-Indian culture, conquer a King or Emperor title in India) at once.
To be able to wage wars without getting world coalition, I released some far away useless duchies (Mercia, etc). I was about to attack Malwa but with > 90 % threat, France, Byzantines, HRE and Venice would all have went to war. Such coalition could not be handled.
I also tried to simplify vassal management by giving some king title to some vassal. My arch-bishop were too strong to be easily managed so it was convenient to give them some king (Syria was de jure liege for their title) as canon fodder. This policy worked for a while but, still, the said king was growing in power and could have been a problem in a game with no time limit.
Nonetheless, I managed to get foothold in India and ultimately convert Sindh to orthodox.
7. Last steps
Once done, it was quite impossible to wage any war due to threat levels. Any expansion was now tied to weddings and the dynasty sprouting over Europe. I was counting on getting the massive Bulgar kingdom but timing was wrong, even with a very active management of the inheritance (useless heirs made monks, commiting depressed ruler suicide)
So, starting in 867, by 1453, I reached 622175 score, and growing fast (you can see the inheritance next: next Kaiser of HRE and King of Bulgaria).
I think that is my best score in this game but usually I start from lower rank (count, duke).