T*A Defense Guide

First and foremost, whatever you do, do not sit in Cav front. Make it a habit to swap to a preferred frontline every time after you do any military action.
A frontline that a large number of people prefer is RNG Wedge.

Now that we have that out of the way, let's talk about some actual defense tips.

Wall

Always have your leader set to a military hero, and on your wall use a mix of heros for INF, RNG, and CAV.

For f2p, these heroes generally will be: Rose Knight, Bombin Goblin, Demon Slayer, Child of Light, and Snow Queen.
This gives hero perks of: INF 70ATK 50HP, RNG 70ATK 50HP, CAV 70ATK 100HP

For p2p (without 11k heroes), a common layout would be: Songstress, Lore Weaver, Berserker, Watcher, and Storm Fox.
This gives hero perks of: INF 100ATK 125HP, RNG 100ATK 125HP, CAV 100ATK 125HP

If you have 11k heroes, what are you doing reading this guide? Just put 11k heroes on your wall and call it a day.

For your wall familiars, if you don't have the p2p fams unlocked, use Evil Weevil, Aquiris, Terraspike, Jaziek & Magmalord.
If you have the p5 fams unlocked (make sure their skills are maxed), use Bon Appeti, Blackwing, Gargantua, Jade Wyrm, and Evil Weevil.