Siege

When a campaign reaches the city walls the character of warfare changes completely. Rifled accuracy makes defense extraordinarily powerful: defenders behind walls pick off attackers at three to five hundred yards, and direct assault is suicidal.

Starvation and supply cutting: Sieges are usually decided this way. Surround the city, cut the supply lines, wait. Time works against the besieger, whose army degrades in the field.
Formal approaches: Trenches dug progressively closer are more essential than they were in the smoothbore era, not less.
Bombardment: Still matters and costs more than it used to. Defenders suppress gun crews at longer range, and counter-battery fire from a wall is genuinely effective.

And the defenders are motivated past anything an attacker can match. They are defending their actual city, their home and family; civilians take part; surrender feels like the deepest betrayal of everything that matters. Sieges in the Known World are fought with desperate determination by the people inside the walls. See Why City-State Wars Are Rare#No mechanism for total victory.

See also