Unak Grenadiers unlock at Castle LVL. 6. To increase their max level, upgrade the Unak Citadel.
Compared to the Archers and Storm Maidens the Grenadiers go for less frequent, stronger shots. And in all honesty these are the ultimate in ranged death. Outdistancing even Ranger Titans, a focused line of these backed up by a decent Titan and carefully controlled by a decent player can shatter armies of a higher level that outnumber them without even taking any troop losses (barring lucky spells). Archers, Storm Maidens, Swordsmen, Spearmen, Imperials, non-boosted Goliaths, Skeletons and every type of melee Titan will get utterly shot to bits by these before they can do anything, unless they vastly out-level the Grenadiers. Smaller number of Panthers, if carefully targeted, can also be handled. Grenadiers are, bar none, the ultimate economical attack troop that can handle nearly every known situation.
On the defense is a bit more tricky. Their slow movement speed makes them rather vulnerable to spells, and a Titan with decent fire defense (particularly a Ranger) can distract their firepower and plow through them systematically when backed up by more Grenadiers. Still, this is a viable option.