That's how it works in M2TW. The general, once married to your princess, acts like an adopted son or man of the hour promoted genral. He isn't part of your main bloodline (not an issue outside of the Stainless Steel mod) but he is capable of having children, becoming King and so on.

The stack gets disbanded, which is a major blow to the HRE as their recruitment is very limited in the starting regions for this campaign. That's why they were going for Hamburg, the AI has gold if you play on a difficulty higher than Medium, but it doesn't have enough stone castles to spend it.

Taking the HRE's citadel will cripple them completely. What you need to watch out for is the Teutonic Order's expansion to the east. If you let them sit back for 20 turns you'll have to deal with very strong stacks later on.