Citadels are the best way to make clan chests. They can be done without any losses and their chests give Tar. Here are the Basics of How to kill a Citadel of different levels and also some more advanced tricks to do them without any losses. The actual numbers can differ depending on your personnal bonuses and researches.

200 T2 Catapults Ranged Units Flying if you can

2200 T4 Catapults Melee units Flying if you can

400 T3 Catapults Mounted Units
Flying if you can

4000 T6 Catapults or 150 T9 merc. Catapults
Melee Units
Flying if you can

1000 T4 Catapults Melee Units
Flying if you can

3200 T5 Catapults or 100 T9 Merc. Catapults Melee Units
Flying if you can

250 T9 merc. Catapults Melee Units
Flying if you can

There are a way to do losseless citadels up to lv20. Here are few tricks. The amount of troops can differ due to different player researches and bonuses. First of all, you need all the possible "monster health" Hero talents to do this, and preferably an "army health" clan bonus. In case you still fail, you may have to level your Ingrid captain or craft some "army health" equipments. Always use Ingrid, Sophia, and Ramses if you have him.

LV. 10 // Also works with 200 catapults2 / or 1600 archers3 instead.

Lv.15 // Also works for Lv.10

Lv.20 // Also works with Lv.10 and Lv.15 // Replace the 60 Griffins7 for 200 Griffins5 if you haven't unlocked G7 // Replace the 1200 Vultures6 for 2000 Vultures5 if you haven't unlocked S6.

Lv.20 *Cursed*