You will be able to revive 90% of your fallen troops using Gold if they died while attacking another player or a monster. If the troops died while defending a location from an enemy, you will be able to revive 75% of the fallen troops using Silver. As an alternative to Gold and Silver, you can also use Sacred Potions, which are produced by your Temple, to revive any fallen troops.

The cost of reviving troops, as well as the cost of producing Sacred Potions, depends on the level of your Temple. Some tournaments affect either the currency that can be used to revive troops or the cost itself. Ruthless Slaughter allows you to revive all troops that died in an attack using Silver instead of Gold, while Thirst for Battle decreases the cost of reviving troops that died while attacking other players.
Please note that if only one unit dies during an attack or while defending, you won't be able to revive it.