The Khmer is a powerful siege civilization that puts a lot of emphasis on two units: Scorpions and Battle Elephants. Their Scorpions receive both a team bonus of +1 range and their unique Double Crossbow tech, allowing them (and their Ballista Elephant unique unit) to fire a second projectile for a potential double damage. Meanwhile, their Battle Elephants are fully upgradeable, move 15% faster, and benefit from their unique Tusk Swords tech; with Husbandry research, Khmer elephants are fast enough to catch up to foot archers or serve as economy raiders.
Their Ballista Elephant unique unit also has the ability to cut down trees, and is the only unit capable of doing so in the Castle Age. This allows them to catch enemies off-guard in Black Forest maps. Otherwise, it is basically a more durable version of the Scorpion with less range and attack. Because of the unique combination of being a cavalry unit and siege weapon, the Ballista Elephant is the only siege unit that can be healed by Monks, which are a welcomed addition to Khmer armies due to their costly but very durable units (despite the Khmer's poor monastery tech tree).
Khmer have access to every siege weapon in the game except for the Siege Onager.
Despite their lack of early game economy bonuses, they are an exceptional early game civilization due to their civilization bonus of ignoring building prerequisites, saving up to 425 wood when executing popular strategies such as the fast castle rush (with only Mill + 4 Houses, skipping the Lumber Camp, Market, and Blacksmith). Furthermore, it encourages flexibility in gameplay and strategy, and allows the Khmer to be completely unpredictable and ignore conventional build orders to can catch the enemy off-guard.
Their ability to garrison villagers in houses to protect them from raids (although they don't fire arrows) allows Khmer to save villagers where other civilizations might lose them. It bears remembering that walled-off areas are still accessible to Khmer villagers as long as a House is used as part of that wall.
While the Khmers are a very strong siege civilization, they have severe weaknesses as well.
Their Monks and Monastery tree is one of the worst, as they don't have access to Atonement and Block Printing, making their Monk rushes incredibly weak. To make things worse, they don't have Heresy and Faith, making their expensive units such as the Ballista Elephant and Battle Elephants incredibly vulnerable to enemy Monk conversion (especially against civilizations with strong Monk rushes such as the Aztecs and the Spanish).
Their cavalry line is very reasonable in the early/mid game, with all upgrades available for a Scout or Knight rush. However they lack Hussars, Paladins, and Camels, making their late-game cavalry (other than Battle Elephants) underwhelming and is completely outmatched in mixed cavalry (Knight/Camel army) engagements. They are forced to rely on Battle Elephants, which despite a move speed increase cannot keep up with the mobility of full cavalry armies.
Their infantry and archer lines are both weak. They lack the Champion upgrade, as well as Squires and Plate Mail Armor, resulting in incredibly weak halberdiers in late game. They're missing the Arbalest and Thumb Ring, and despite having Parthian Tactics their Cavalry Archers fight poorly due to their low 50% accuracy without the Thumb Ring upgrade. They still have effective Pikeman against Knight rushes and Hand Cannoneers to counter mass infantry.
With poor Halberdiers, no Hussars, and no Champions (the default trash-killer), the Khmer are incredibly weak in late game trash wars when gold is scarce.
The Khmers' ability to build buildings or advance ages without prerequisite buildings is difficult to use well. It can be incredibly strong, but it can also backfire hard. For example, if the Khmers attempt to go for a fast Castle Age for a Battle Elephant rush without building other important buildings, they will not only have a weaker economy, but can also be punished hard by a Feudal Archer or Scout rush (due to inability to produce Spearmen and Skirmishers), or even a Knight counter-rush. The Khmers should use this civilization bonus wisely and adjust to their strategies based on civilization matchups, maps, and situations.
The Khmers are a very unique civilization on their own, since they do not need to build any pre-requisite building to either advance to the next age or unlock other buildings. This makes the Khmers not only incredibly versatile, but also incredibly unpredictable when it comes to gameplay and strategy. It is possible to go for a fast Castle Age rush within under 15 minutes since the Khmers save a lot of wood and time when advancing to the next Age, although such strategy would leave them vulnerable to enemy raids in open maps. The Khmers can go directly for a Archer or Scout rush in the Feudal Age without the need to build a Barracks, hence allowing the Khmers to close out games in the Feudal or Castle Age relatively quickly. Regardless, when playing the Khmers, it is important to be unpredictable on what you are about to do.
If a player intends to perform a rush with Khmers (independent of which kind of unit they will use), the focus in Dark age for them should be on food entirely and use their initial wood on three houses and two farms ( as they don't require a mill to build farms). This will save the time required to create a lumber camp and a Mill and also while other civilizations will have to collect 100 wood in order to create a lumber camp for aging up, the Khmer villagers will only have to collect 500 food and the lumber camps should only be created when the trees near the town center are chopped out.
Unlike other civilizations the Khmer houses are capable of garrisoning villagers. This ability enables for them to minimize loses when an opponent is raiding their towns, so it is advisable to create houses near to the working areas of the villagers and also around the farming areas. This bonus can also be valuable for turtling as the houses can be used as a wall substitute while allowing villagers to still pass them (by garrison and degarrison on the other side). A useful application of the bonus can be seen here.
One of the biggest strengths with the Khmers is their strong Scorpions. Since Scorpions are considered to underused by most civilizations, this makes Scorpion rushes very viable for the Khmers due to their team bonus granting Scorpions extra range and their Double Crossbow unique tech. In fact, a fully upgradable Heavy Scorpion can actually outrange a fully upgradable Siege Onager (except Korean ones) if the Scorpions are microed properly. The Ballista Elephant is basically a tankier version of the Scorpion with lower range and attack, but the natural tankiness itself makes them very threatening against massed infantry and archers. Their ability to cut down trees makes the Khmers one of the top tier civilizations in Black Forest maps, allowing them to catch enemies off-guard when playing defensively. Ballista Elephants should be well-protected though, as they still have their typical cavalry, siege weapon, and elephant based weaknesses (especially Monks), so they need to be protected based on the enemy's army composition.
Across from their powerful siege, the Khmers have one of the best Battle Elephant rushes in the game. Having access to their typical cavalry unit upgrades including all the Battle Elephant Blacksmith upgrades, Bloodlines, and Husbandry, their Battle Elephants also move 15% faster, making their Battle Elephants essentially raiding units and more threatening to enemy foot archers. In fact, Battle Elephant rushes should be considered over Knight rushes since despite being a bit costly, their high mobility combined with the Battle Elephant's tankiness makes them incredibly threatening to towns since Battle Elephants also do bonus damage against buildings. The second unique tech, Tusk Swords, makes their Battle Elephants even more threatening as well.
The Khmers are also very solid on water maps, since they have access to every essential naval units and upgrade, only lacking the Two-Man Saw and Heavy Demolition Ship. In fact, thanks to their civilization bonus not needing to build any pre-requisite buildings, the Khmers can easily skip out on Mill or any other military buildings and simply focus on building a Dock and Blacksmith with Fishing Ships and Fish Traps as their main food income.
In general terms, the Khmers are essentially a fast-paced civilization with emphasis on unpredictably. They can perform a variety of strategies (scout rush, archer rush, fast castle, knight rush, galley rush) better and faster than most civilizations. Those who enjoy playing civilizations with versatile playstyles such as a Byzantines, Malians, and Chinese can find themselves home with the Khmers (especially for players familiar playing as the Chinese since the Chinese share the same high skill cap as the Khmers).
The Khmers have a good variety of teammates that would love the extra range for their Scorpions. In particular, civilizations with relatively strong siege line, such as the Mongols, Celts, Slavs, and Teutons can take effective use for the extra range for their Scorpions. Some of the said civilizations have team bonuses that are greatly beneficial to the Khmers. The Mongols extra line of sight for their Scouts allows Khmers to open up for a Feudal Age Scout rush. The faster workrate from the Celts is a lovely blessing for the Khmers since the Khmers already have a strong siege line.
In particular, Slavs and Teutons synergise almost perfectly with the Khmer. The Slavs' giving military buildings extra population have great synergy with the Khmer's quirk of garrisoning villagers inside houses, thus allowing forward bases of military buildings and Houses at the same time, that can potentially be built close to enemy bases in order to harass and exploit nearby resources. Slavs will, in return, have greater use for their cheaper Scorpions and can somewhat better counter the Monks that can convert their Boyars. Finally, the Teutons' conversion resistance and their strong monks and offset the Khmer's weak Monk line and Battle Elephant vulnerability to conversion, while Ironclad-boosted Scorpions with + 1 range can better cover their vulnerabilities against Ranged Cavalry, such as Conquistadors, who wear down Teutonic Knights and Paladins. An alliance that combines Slavs, Teutons, Celts and/or Khmer (at least 3 of them) might prove to be extremely hard to beat (though a Teuton-Slav-Khmer alliance will be better protected against Monks, whereas Celts will allow the team to better exploit their siege).
The Chinese will love the Khmer extra range for their Heavy Scorpions, noting that the Chinese mean weaknesses for their Heavy Scorpions is the lack of Siege Engineers, and fully upgraded Chinese Heavy Scorpions with the Rocketry technology will make the Chinese Heavy Scorpions more threatening. If the Khmers wish to utilize their lightning bruiser Battle Elephants, the Huns are also a great ally. Since the Khmers are the only Southeast Asian civilization to have Hand Cannoneers and Bombard Cannons at the same time, the Turks faster creation speed for gunpowder units is also a nice addition.
When the team intends to perfor a Springboard tactic, Khmer can be used as both as a spring or a springboard, but it should be preferably used as the spring as they can age up quickly to Castle and Imperial age while having also very powerful battle elephants and scorpions as core units and Gunpowder units for sieging.