Hiya, would you mind narrowing down what are you looking for? As it stands, your question covers the whole history of warfare and that can't really be boiled down to something a single expert would be able to answer. Are you thinking of High Middle Ages in Western Europe, Ancient China etc.?

Edit: I forgot to mention what time period and etc.

I'm interested in the medieval ages.

I'm asking about how armies would fight inside the city, when they successfully broke the defending army at the walls and opened the gates, and so forth.

In most movies when a storming army gets into a walled up city, they just run around and slaughter everyone they see, but was this really true? Or did they still fight in battle formations?