Definitely keep the cities. I use to keep them till the AI civs get Nationalism and begin to draft like mad. Place only one defender in it and keep a bunch of attackers nearby to take it back if it flips. This is the more likely, the closer the city is to the enemy capital. Build up your own culture and poprush a library (you're scientific) in the city ASAP (after prebuilding a bit), to get some own culture in it and to raise your overall culture.
Generally, the Germans are a good civ to hit from the start. Cheap barracks, archers and spears from beginning, 1 tech apart from swordsmen. That makes a solid advantage for an ancient warmonger. Later in the game their traits are rather mediocre. The other ages see other civs dominating, like Chinese and Japanese with their good medieval UUs. The Germans get a second good chance only with their late industrial UU. So hurry up and secure a big slice of the pie in the ancient age.
Edit: Ah yes, keep the AI's alive, but only till the mid of the medieval age (Navigation), or till they cease to give you something valuable for peace. Remember, every city they build is worth 30 shields and 40 food (or even more) for a settler. So don't frown but be glad if they plopp their cities near your border. It's for your own good
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