The nation of Aachen is a free-city in the northern Holy Roman Empire that boarders the Burgundian lowlands, Liege, Colonge and the Elector of Trier

Development
Aachen starts as 12 development with means to grow the development quickly, being a free city they get a bunch of events to improve the development quickly and they boarder the Lowlands which are some of the richest provinces in the game

Government Type and Leaders
Aachen starts with a Stadtrat which means your leader is going to have random stats at the beggining of the game (most likely good) with a random age between 30 and 60 (?)

You are a free city, which means if you expand past one province you become an Oligarchic Republic, use the usual Republics' strategy that is re-elect your leaders until your at ~40 Tradition then spam elect the 1/1/4's, 1/4/1's and 4/1/1's its worth it for the many 6/6/6's you'll accumulate over time.

Playing Tall

Every free-city, has two choices, Play tall n' Small or Play Wide with Pride.

Playing Tall is very easy for a free city, as you basically won't have to worry about being attacked because Mr. Emperor has basically Guaranteed your independence, so if you feel happy to sit tight as an OPM for many a year then go ahead and dev and tech up.

However you can still play expansion, find a good ally or two and expand into the lowlands and get a port, then move your capital into the lowlands to avoid pesky rebels and you can colonize and grow, all the while developing and improving your empire for the better.

Playing Wide

Playing tall is a much harder task for little Aachen, local allies are hard to come by and if you want to expand into the lowlands your gonna need a French ally, and if you plan heading south your gonna need an Austrian willing to help your duchy, Eastward expansion would be eased by a Brandenburg, Bohemian or Danish ally. In the end upping your dev by playing tall for the first hundred years will help Aachen gain allies and flourish.

Humiliation Wars

I'd advise this to every nation in the empire, and most regular nations, if your locals have smaller or near to same size armies, invade them with the humiliate CB just so you can "Show Strength" which gives you ~10 Prestige, ~50 Power Projection and One Hundred of Each Powerpoint, which is very useful for small states like Aachen and even bigger states like Austria, England or France. And Perhaps even Ming. In the end its a good way to stay up-to-date in tech and development.

Remain Catholic, Convert to Protestantism? Is Calvinist the correct way?

This is mostly up to your style but I have a few suggestions. Because of your position, it will hinge on the other nations of Europe, if France doesn't go Protestant, but Brandenburg or someone else would ally you if you were Protestant it may be better to go Protestant.



Around this time the Netherlands are going to declare independence and they will be desperate for allies, so I'd suggest allying them and help them fight their oppressors, They'd make a very good Meatshield ally, copy their religion, most likely Protestant or Reformed and your good to go.

The League Wars

This tricky divisor of Europe is a problem for you, as a free city, you want either your faith or Westphalia to kick in, so either stay out of this conflict or join your faith if you think they have a chance to defeat the others.

Most of the time league wars are not worth spending your men on so I would suggest 80% of the time to stay out of it for the sake of your nation and people

German Ideas
Aachen starts with German ideas which aren't that bad their just worse than Prussian ideas and often overshadowed but from here on out its German ideas so get used to them and learn to love 'em

Infantry Combat Ability
While it may not be Prussian or Dutch +10% is better than none, and it will help you win battles even if your outnumbered (sometimes)

Technolegy Reduction

I love tech reducion costs they make teching up so much more entertaining and the German benefit to that makes you have the Tech of europe and past it, because of this benefit a good tall player will never fall behind on tech.

What Ideas do I choose?

Because your a duchy with urgent needs for allies, go diplomatic first for the diplomatic rep. and extra diplomat. combined with more slots, diplomatic will make it much easier to expand

Follow the Diplomatic ideas with Offensive, Defensive or Quantity, you need an idea group to defend your self so choose wisely.

Finish the first three groups with Economic or if you have a coast colonization, you need a boost to your econemy and these are two easy roads to take.

Westphalia?

Little Aachen can form mighty Westphalia which has a diffrent idea group that may fit your needs, aswell has a good prestige boost and some nice perma claims in the north HRE, however Westphalia needs some provinces from the local big nations like Brunswick and more importantly Colonge, and you have to do it all without making the emperor hate you so he makes you an elector, its hard and relies on chance, but it may be worth it as you become Kingdom rank and an elector.