Neutral estates in stronghold 2 cost 100 honour. To purchase an Estate, click on the flagpole and click the "Cost 100" button.
Once purchased, unlike Stronghold Legends you may build most buildings in that estate. Carters will automatically deliver goods to your estate on a regular basis. You may build more carter posts and choose custom goods for it to send instead as well as the amount per trip. You will gain honour from estates, as well as tax so they are an ideal way of helping you progress in the game. It is also a good idea to put troops next the the estate flagpole of any estates you have, as any enemy troops can go near it and it will countdown from 2000. The more troops the enemy has near the flagpole, the faster it will count down.
Once it has reached 0, the estate will be taken by the enemy. The same concept applies to you - if an enemy has an estate then you can take it by killing all enemy soldiers near it then leaving your troops there long enough.