It is now 1506 in my HRE campaign. My conclusions so far are: playing HRE is rather similar to MTW. There is a peak of trouble you have to survive when most of your neighbours come after you. That peak for me was around 1250 when I had war with Denmark, Poland, Venice and Sicily and Milan join enemyship. If you manage to get through it nobody really can endanger you.

Spain, England and Hungary attacked later but that was only annoying, not dangerous. Nevertheless I have to keep at least 8 field armies at the borders because there are no natural barriers except against England.

Units
German units are of mixed quality. There is no really extraordinary performer. In the early and middle game you have Spear Militia and Armored Sergeants and the normal knights, nothing special.

Of course you need Pavise Crossbowmen (castle) and Halberd Militia (city) in the middle game. After I got the halberds I used Armored Sergeants only in second class armies.

Zweihänder are a bit better than foot knights but not that good as I thought in the beginning.

If cavalry would be easy to use and without bugs HRE would be much stronger. Gothic Knights (MTWs darling) are great shock cavalry (in theory) and Mounted Crossbowmen and Reiter are fast and deadly from the distance. But more often than not they failed to charge correctly so playing hammer and anvil is not always possible. If caught in melee Gothic Knights are as useless as every other cavalry unit I know so far.

There are a lot of canons available. With it, crossbowmen and arquebusiers horse archer/missile armies were no real match which was a bit surprising to me. You have substantial losses but invariably win the match. More dangerous (to my taste) are foes with strong infantry like the Aztecs for example. It's a pity that Landsknecht pikemen are only available as mercenaries for they would be a good core of the armies.

Cities/castles
Also in the late game there is only one thing you need: money. Therefore I transferred most of the castles I conquered to cities. My "military triangle" was Hamburg (castle), Magdeburg (castle) and Frankfurt (city). All were build up as fast as possible. Hamburg is for Zweihänder, Pavise Crossbowmen and Mounted Crossbowmen (which are not build in stables but in the archers lab), Madgeburg for Gothic Knights and Reiter, Frankfurt for Arquebusiers, Halberd and Pike Militia. You can form armies easily in the middle of the three. Cannons are build in Aarhus (you need a city with a harbour for building carracks to reach the new world, and the last building for cannons improves also your ships, so it's a good combination to build canons in a harbour town). Beneath the named strongholds I did not develop military structures. I only use what I find when I conquer foreign land. Let your enemy spend the money, you need it for the many troops you have to field.