Place walls to protect your own buildings against ground attacks. A single wall only costs 150 lumber, but will not last long. Upgrading one section of the wall costs 800 lumber. Start by creating a basic wall around your entire city. Then, surround your most valuable structures, such as the castle, with a level 2 wall. Save the rest of your lumber for defenses that are effective against dragons. Use Troll Towers and Drakecapults for defense, as well as Goblin Silos. They will fire off ranged missiles that do large amounts of damage to dragons and ground units. However they take some time to reload.
Dragon scales
Dragon scales are required to level your dragons. They can be obtained by completing the daily challenges.
Raiding
When you raid another camp, you can see an overview of their buildings. Take note of where their defensive structures are located (Drakecapults, Troll Towers, etc.). Target those buildings first to remove threats to your dragons. When you select a dragon, tap a tower to have it immediately attack that specific target. You can also do this at when a dragon switches targets. Ground units are recruited through a summoning portal. Although they are not as powerful as your dragons, they will aid your offense. You can eventually build a Boot Camp to level up your units and strengthen them. When deployed, have your dragons and ground units work together. Use your dragons to destroy walls that are out of range of defense towers, and allow your ground units to invade. Then, have the ground units keep enemy defenses busy to allow your dragons to operate freely.
Resources
Lumber and iron are important resources. They are generated by iron mines and lumber mills and stored at the lumber depot and iron depot. Keep those buildings upgraded to gain resources faster and in larger quantities, and store more of them. Participate in single player raids to gain more resources. They feature attacking another camp to steal their resources. This can be done against other players or in the pre-made single player missions.