The Defense Tree will generally be your 3rd priority (after The and ) when it comes to finishing their respective Essential Techs.
In the Defense Tree you will find a variety of Techs that make your Walls, Traps, and Troops stronger when you are being attacked. There are some key Techs in this tree that you 100% absolutely should have maxed before participating in in any serious manner. Here you will also find Techs to reduce Troop Upkeep and two very useful Techs dealing with healing your troops.
Defending Troop Attack, Defense, and HP (including Advanced and Super):
The levels of Attack and Defense combined provide an extra +110% to both of those stat blocks when defending your city. This coupled with the +50% HP you can get in this tree gives you a huge advantage when defending since in the Military tree you can only get +50% Marching Attack and Defense and no Marching HP buff at all. These Techs are so important, I cannot stress this enough. You will get hit at some point in SvS, lag will eventually get you. You want this advantage for your side when it happens, big time.
Advanced Medical Power, Medicine, and Advanced Pharmacy:
These three Techs are vital to everyday life. Whether you're running bosses or setting things on fire, buffing your Hospital Capacity and Healing Speed, while decreasing Healing cost are the right choices to make. Honestly, you should max these before maxing anything else significant in this tree as they aren't especially expensive or lengthy Techs to research.
Marksmanship and Advanced Archer Tower Attack:
When you are defending, you get an archer tower to help you in the fight and your enemy doesn't. Might as well buff the dam thing then, right? While not a super high priority, you should probably get these maxed out at some point as they're rather cheap and quick.
Trap Quality, Trap Power, Adv Trap Power, All the different Trap Attacks:
If you're thinking about building a bunch of traps for your defense, you absolutely need to work on getting all of these maxed. I would say the most important ones would be Trap Quality and then Trap Power and Advanced Trap Power. Keep in mind that if you choose not to build traps (yes, you don't have to have traps) you don't really need these at all and might as well ignore them for now.
Cooking and Super Cooking:
Knock down your troop upkeep a bit. Doesn't really help that much when you're troops are still eating millions an hour. I don't even bother with these Techs.
Fortification and Engineering:
These guys are pretty cheap and quick so just knock them out. They aren't super important as you can see in the photos above, my engineering was at level 2 😅. Don't worry, I maxed it as soon as I noticed. These Techs basically just make it take more hits to Force Port (fp) you.
Blacksmithing and General Revival:
Blacksmithing is great because General Gear takes forever to collect the materials for and you want it done as fast as possible and this will help speed that up. General Revival is pretty pointless since Revive Stones are pretty cheap.