I play as CSA. To defend Manassas I left 8 elements, 2 Generals and 2 supply wagons inside the city (posture defend/defend). They were entrenched only - no fortification of the city. To my big surprise the USA moved a big army stack (PWR >800) onto Manassas, but without attacking, sieging only. In the next turn my troopers surrendered without fighting.

I did not have expected that a garrison just surrendered without fighting - even they were good supplied and with plenty of ammo.
I set back the game to the turn before. But this time I set my troopers inside Manassas to defend/hold at all costs. In the next turn they did not surrender. Instead a breach occurred. Another turn later all my troopers surrendered, again without fighting.
Now I ask me how to stop the blue wave. It is my very first game. I assume selecting the CSA for a newbie player like me is a special challenge. My central question is whether it even makes sense to leave troops in a city, even if the city has fortifications? Isn't a city a death trap then?
Thank you very much.