The Khmer is a powerful siege based civilization that puts a lot of emphasis on Scorpions. Their team bonus grants Scorpions +1 range and one of their unique techs, Double Crossbow, allows their Scorpions and their unique unit, the Ballista Elephant,to shoot an extra projectile, hence increasing their damage output. The Ballista Elephant also has the ability to cut down trees, thus catching civilizations off-guard in Black Forest maps during the Castle Age. In regards to their unique unit, the Ballista Elephant is basically a more durable version of the Scorpion, but with lower range and attack. Because of the unique combination of being a cavalry unit and siege weapon, the Ballista Elephant can be healed by Monks, an unusual attribute for a siege weapon. In regards to their siege weapons, they have access to every siege weapon in the game except for the Siege Onager. Their Battle Elephants are also very threatening, being able to move 15% faster and have a unique tech, Tusk Swords, that boost their offensive prowess, allowing their Battle Elephants to be effective raiding units and close the gap on foot archers. The Khmer also have strong defensive tools as well, allowing Villagers to garrison into Houses if they are being attacked, thus saving their lives from potential devastating raids (although the Houses themselves will not fire back with arrows).
In terms of their tech tree, they are essentially a "jack-of-all-trades" civilization in a similar vein like the Byzantines, Chinese, and Malians, having the essential units in their tech tree. In fact, one of their civilization bonuses does not require buildings to advance to the next age or unlock other buildings, thus not only allowing the Khmer to save wood and time, but encourage flexibility in terms to gameplay and strategy. In fact, this civilization bonus allows the Khmer to be completely unpredictable and versatile when it comes to their strategies and can catch the enemy off-guard.
While the Khmers are a strong siege civilization, they have noticeable weaknesses as well. Their Monks and Monastery line 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, despite lacking Hussars and Paladins, but their infantry and archer line is a bit weak, as the Khmers lack Squires, Champion upgrade, and Plate Mail Armor for their infantry and the Arbalest and Thumb Ring at their Archery Range. However, the Khmers do have Halberdiers for a cost-effective anti-cavalry counter, and Parthian Tactics, Hand Cannoneers, and all essential archer upgrades at the Blacksmith for anti-infantry capacity.
The Khmers are a civilization that is also incredibly difficult to play properly, since while the ability to build any buildings and advance to the next age without any pre-requisite buildings is very strong, it can also backfire hard if used incorrectly. 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 their Battle Elephants can essentially be punished with Pikemen or worse, enemy Monks. 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. For example, if facing a civilization with strong cavalry such as the Huns or the Berbers, it doesn't hurt to build a Barracks and counter them with Spearmen.
If a player intends to perform a rush with Khmers ( independently which kind of unit he or she will use) the focus in Dark age for them should be on food entirely and use their initial wood on four houses and a mill, this will save the time required to create a lumber camp 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. Sometimes even the Mill will not be necessary to build, particularly if the player manage to found early their herdables and their boars ( or their equivalents), and when this happens they can build a farm instead.
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 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.
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 "jack-of-all-trades" civilization with emphasis on unpredictably. The Khmers aren't simply just a siege and Battle Elephant civilization and have all the tools needed for different variety of strategies. Those who enjoy playing civilizations with versatile and diverse tech trees 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.