When you have built or upgraded your Estate, you will have a quest to capture an Outpost. The report on the right shows what you need to defeat for an unoccupied outpost. Since the castellian is super slow, even with ruby horses, it isn't going to affect what you bring. macemen will be strong enough to take out the Armed Citizens if you take all 3 flanks, with 3-6 losses. However, if you have Two-handed Swordsman, you can win without taking any losses, or at worst one, with the same attack. Once you have gotten the outpost, it is wise to switch the troops to defend, such as Halberdiers and Longbowmen for the first few hours so you don't lose the outpost right away. Then you can bring whatever troops you want. It is favorable to bring attack troops to one outpost, so you have more troops at one place to attack other players, Nomads, Samurai, Foreign Lords/Bloodcrows, and Berimond camps. You will want to have all 8 food outposts if possible, so you can have more troops, however, even 6 food outposts can suffice. I (IaLL/kuku9999) went to an 8 food, 8 wood, and an 8 stone, so I could build quicker everywhere, with my food OP being stone as the secondary resource.
In terms of capturing someone else's Outpost, make sure you have either agreed you want to lose that outpost, so you can find a better one close by your main castle, or are switching outposts in general by both players. Attacking other outposts is not a good idea, as that may cause wars, unless the person your are attacking are not in an alliance, or in a very small one (no more than 5 people).