This is a guide based upon my experiences playing as the duchy of Gwynedd. Starting off as a Welsh player, you are confronted with a very existential threat: England, the juggernaut of the British Isles, can only be occupied so long with the three way war between Hardrada, Godwinson and William the Bastard. Wales offers a tempting target to the English, but within only a hundred years or perhaps even earlier you can make Wales badass enough to beat the crap out of both England and their allies, in my case the Holy Roman Emperor himself. My experience started as of the Hastings era.
Unification of Gwynedd and DehuebarthEdit
When playing as a Welsh faction, it will be more difficult if you play as one of the counties in Dehuebarth duchy. Military conquest will take longer, and marriages (with convienient assasinations) will most likely be your best bet for quickly uniting Wales. If you start off as Gwynedd you have the advantage of having 2 counties under your direct control, as well as a third county under the control of a vassal. My first task was revoking this county, under the intrigue tab. You need at least one vassal to support you in this, if so you can try to revoke the county. Either the count will agree, or he will rebel and you can defeat him in battle. Once you gain direct control over this third county, uniting Wales by gaining control of the Dehuebarth counties is your next priority. As they are disunited you can easily pick them off one by one.
Blitzing Ireland and stealing CornwallEdit
After Wales is united, you have a great deal more power than initially, but England is still a powerhouse that can squish you. You need more troops and territory, but where to find it? Easy answer: Ireland. Ireland is extremely fractured, where virtually every count has only one county holding and only one or two Irish allies. In another game of mine I conquered Ireland as the Greek duchy of Cyprus, which has only 2 counties, so as Wales it shouldn't prove too difficult. I started in Munster and quickly carved up Ireland by picking the divided counties off one by one with my superior resources and manpower. NOTE: if you do not wish to incur the trucebreaker penalty and do not want to halt the momentum of your conquest, try and initially attack any Irish nobles that own more than one county, as after you win you have a ten year period where atacking them again will incur a penalty with all Christian rulers and a prestge penalty as well. Once Ireland is conquered, you have enough strength to serve as somewhat of a deterrant to England, and I would say that definately so against Scotland.
Here's where I got lucky. Oddly enough in my game Hardrada initially won the war and formed a really scary looking North Sea Empire (not what it was actually called, but that's what it essentially was). The Yngling Royal House of Norway and the Normandie Royal House kept swapping control of England, until eventually the Duke of Cornwall (who was part of the kingdom of Norway while the rest of England was rulled by the Normandie family) rebelled against his Norwegian liege. If Cornwall had been part of England the king would have easily crushed him, but as Cornwall was currently part of Norway it was far enough removed to make Norwegian assault difficult. I siezed my chance and took over Cornwall and Devon, thus completing the entirety of the Kingdom of Wales.
With the whole of the Kingdoms of both Wales and Ireland under your control, you have become a major power player. You can go where you wish to build your power up to even greater heights. I would strongly recommend constructing buildings in your holdings and building up your Welsh longbow retinues, which are quite good at defense from my understanding. Economic buildings first to build up your income, military buildings later. My game is at this state currently, it is 1250 AD and I have kicked England's ass in a series of small scale wars for counties and duchies which I have taken from them. I tried to marry into the ruling families of England and Scotland to try and take over the whole of both, but I'm still struggling on this point. I use assasination liberally to keep my vassals under control, although this is a rather expensive strategy to employ. I have medium crown authority in Wales and Ireland, and I set the levy and tax laws to their minimum levels for the extra opinion bonuses I have with my vassals. I have an Antipope at the moment, although I cannot necessarily recommend it yet because it is my first time using one. I put him in place because the then current Pope excommunicated me at the behest of one of my vassals, and I wanted to no longer be excommunicated and also get rid of my troublesome vassal. Long story short, Antipope is in Wales, he likes me very much, I can excommunicate virtually everyone in my realm, and I have yet to be invaded by a pro-Pope army, although this is a possibility for the future (I think).
Hope this gets read by someone who finds it useful, kind of bored at the moment and needed something to do. Sorry if you read this and did not find it useful in the slightest.