The Domrey becomes available later than the Catapult and has nearly twice the Production cost, but it boasts 10 more Combat Strength and Bombard Strength. More importantly, it can move and attack on the same turn without the Expert Crew promotion (or a Great General), and the Domrey's Zone of Control means that three of them can place a city under siege if positioned properly. A few Domreys with melee or cavalry units to back them up can quickly punch through medieval city defenses, allowing the Khmers to expand their empire through conquest in the middle stages of the game.
The downside to Domreys is their reduced Bombard Strength compared to Bombards (45 vs. 55), to which they cannot upgrade. No other units can upgrade to Domreys either, meaning that you'll have to train as many of them as you need while they're available. Also, since Domreys are siege units, there are no Policy Cards to aid in building an army of them any faster. The Khmer are a civilization that already has trouble with Production since they require a lot of infrastructure pieces (Holy Sites, Aqueducts, etc.) to utilize their bonuses, so the Domrey is strong yet hard to use practically. You should focus Production to build this unit en masse only if you go down the path of a warmonger (which is very unusual for the Khmer). Otherwise, if you just want a regular game aiming for a Cultural Victory with a Religious Victory backup, there isn't much justification for building a lot of Domreys; since they are siege units, they are much worse at defending your territory than other, cheaper units.
If there aren't enough horses for your cavalry, why not use the mighty elephant to crush and terrify your enemies?
The Khmer joined a long tradition of employing war elephants in their military campaigns. Research suggests that the Indians first used elephants in their military campaigns around 1,100 BCE, tuning what was at that point a farming helper animal and turning its 12,000 pounds of might into sound and terror for their opponents on the battlefield (at least until the invention of gunpowder evened the odds).
The mount of kings, military leaders would ride elephants into the battlefield, using their lines to stampede through enemy forces.
Khmer riders would use the precision of double crossbows to supplement the raw power of their elephants. However this domrey variant adds a bit more sheer force with a mounted ballista to impale the bold soldier not wise enough to run away.
- Domrey simply means "elephant" in Khmer (ដំរី).