What defences to upgrade first?

Here is the order on what defences you should upgrade first:

Wizard Towers Mortars Walls 1 level below max Inferno Tower Xbow Air Defence Hidden Tesla 1 Level below max Cannon Archer Tower Walls Max Hidden Tesla Max

Wizard Towers and Mortars and vital to upgrade as they can do splash damage, self explanatory. Walls keep out attackers extremely well, highly recommended to keep at a good level. Inferno Tower and xbow pretty much decides the battle with hogriders and Golems/Wizards. Air defence = No air what-so-ever. Hidden Tesla is the ultimate element of surprise, if you have a few empty 2x2 spaces to throw off the enemy.Cannons are more important then Archer Towers because Archer Towers cost a lot more and require more time to upgrade but they are both point defence. The only upside to Archer Towers is that they can target air, but if your air defences are up to date you don't need worry about the Archer Towers until most other things are upgraded. Walls should be maxed for intimidation and successful defences. And Hidden Teslasshould be upgraded last as the higher levels they go are very expensive and you have 4 by TH9.