You'll need to use siege units to effectively bring down a city's defenses, then once the city's health is 0 just walk a unit into the city to capture it. Attacking full health cities with normal melee units requires a large dedicated sustained assault.
Thanks. Another question: When I start building something, am I suppose to just move my units around the map in order to wait till whatever Im building is completed? Like if I build a unit of warriors or a monument, do I just move my units around the map in like circles so I can skip turns asap so that that the warriors and monument can finish building asap or is there something I should do while waiting for something to complete building?
A better example to question 2 might be that when I was gonna attack a city and wanted another warrior unit I just moved the current units in circle around my city to faster skip turn and to build them.
Think that's it.