One of the best ways to add Castle Value (CV) to your kingdom is to upgrade your castle. Each time you upgrade, you will receive a sizable CV bonus.

To upgrade your castle, simply click your castle, and select the Upgrade Tab on the left hand side. You will see this:
You will notice that the conditions for upgrading are shown on the bottom of the menu. For each time you upgrade the castle, you will have to pay a Medal and Coin fee.

As shown in the picture above, the cost of upgrading to a Hold is cost 10 Medals and 2000 Coins. These costs will increase with each level you upgrade. Your castle can be upgraded a total of 7 times.