Trying to attack cities.

Hello everybody.

I have been giving Elemental another go and have played the starts to 2 or 3 games and I am puzzled on how to attack cities. I declared war on Magnar (I was capitar). They had only one city and I had 3 but when I sent down my main army and channeler to attack it wouldn't let me. I was at war with them and I declared war on them.

Do I need to research a way to take cities?

One other question about exploring was I had discovered "Rare Resources" but in my whole kingdom all I found extra besides the resources that show up on the map were only twilight bees. Seems like alot of research for such a small payoff and it seems like this world are very very bland. It was a large map.