This is a spin-off of the Austria to HRE chain. The core gameplay is similar, but here the focus shifts toward conquering the lands around the Mediterranean and Black Sea. Personally I did this run by loading an earlier save after forming the HRE and redirecting expansion toward the relevant provinces.

For idea groups, the recommended order is Diplomatic, Influence, Religious, Administrative, Humanist, and Offensive — swap Humanist and Offensive depending on whether you want to lean into the HRE vassal swarm or not.

Your first war target is Bohemia. Rival them early for the power projection bonus, then use the Enforce PU casus belli to bring them under your crown. Call in your allies — you've royal married Castile and allied the electors by now, so this shouldn't be a hard fight. One war won, one junior partner secured.

Unlike the Brandenburg to Germany run where you conquer provinces directly from the start, Austria's early game is built around establishing personal unions rather than direct annexation. Your first two are Bohemia and Hungary — Bohemia through war, Hungary through the Archduke of Austria / King of Hungary event around 1455. If the event doesn't grant you the union for whatever reason, you'll need to go to war for it too — call in your allies, Burgundy especially for a good measure. Two crowns either way, no need to paint the map just yet.

Milan is your third personal union, handed to you through a scripted event. The war itself is straightforward — use it as an opportunity to hit some Italian minors on the side. Three unions at once is the requirement, and at this point you should have all three running simultaneously.

By now you likely have a small OPM subject sitting around — Serbia, Bosnia, or similar — that's ready to be diplo-annexed. If you've played Austria before you've probably already popped this achievement in a previous run. Either way, this is a natural checkpoint to clean it up if it hasn't triggered yet.

The Reformation is a major threat, so crush it fast. When the first Center of Reformation spawns — usually in an OPM — declare immediately even without a casus belli, as long as you won't trigger a coalition. Enforce religion and the center is gone. The next ones tend to spawn in Brandenburg, Denmark, or Sweden. Attack, split off the capital province so it holds the CoR, enforce religion. Repeat for all six centers. Your empire is large enough by this point to handle it.

Completely optional, but if any of the colonial powers present in India — Portugal, Castile, England — are willing to sell you a province in the Bengal area, it's a fun detour. Grab it, invest in the District and Military Administration, and tick the achievement. Nothing critical, just a bonus if the opportunity is there.

By this point your expansion around the Mediterranean and Black Sea should be well underway. If you haven't hit 200 provinces yet, keep pushing into the relevant regions — the Balkans, North Africa, the Levant, Anatolia. This is a natural checkpoint before the final push.

Before you go for this, do your homework. Hover over the achievement in the achievements list to see exactly which provinces are required, then open the decisions tab and hover over the Restore the Roman Empire decision — it highlights every eligible province on the map. You'll notice it includes provinces in Britain as well. If you didn't expand there early, Britain can become a serious problem at this stage. It's possible to ignore the British provinces entirely, but just remember you'll need 425 provinces total within the eligible region to compensate — again, check the decision tooltip to verify. Plan accordingly well before you think you're close.