Who needs siege weapons when you've got yourself war elephants?? Of course there's more to the Persians than that, but the War Elephant is definitely what symbolizes this awesome civilization the most.
I remember in my early days of playing Age of Empires II: The Conquerors, when I picked up the Persians because I've had this difficulty facing war elephants in the campaign mode. After learning of the power of the elephants, the Persians became the civilization with which I first played competitively.
Fast-Developing and Cavalry Civilization
Start with +50 food and wood
The Persians have a great growth rate thanks to their unique traits. This will be much appreciated in the late game as the Persians' heavy focus on expensive cavalry will make them use those resources! Starting with +50 food and wood is a great way to gain an edge in the early game, develop fast, prepare for an incoming rush, or prepare one yourself.
Town Center and Dock have 2X hit points
The main production buildings of the Persians have double hit points. Again, this aids a lot in the early game, because a quick unexpected rush may hurt your town. If your buildings have more hit points, they can hold on for longer, giving you more time to prepare that army to counter the invasion, and survive the rush.
Town Center and Dock work rate +10% Feudal, 15% Castle and 20% Imperial Age
This is the best bonus of all in the long run. An accelerated work rate means villagers and ships coming out faster, thus starting to gather more resources and sooner than many of the other civilizations. This is called exponential growth and it does add up, giving a serious advantage.
"The last civilization in the Middle East to employ war elephants was Persia, who got their elephants from India. The war elephant was a powerful complement to an army, especially against troops with no experience against them. They were very difficult to kill, but remained difficult to control also. If they could be directed into an enemy formation, the enemy troops always fell back in disarray."
The War Elephant is the most menacing weapon of the Persians. It's an incredible behemoth unit of 620 HP and 24 attack when fully upgraded! An army of these units can raid towns to the ground like a hot knife cutting through butter! What makes the war elephants so good when raiding towns is that they can be more effective than the Teutonic knights (Teutons) when fighting in close combat, more effective than the huskarls (Goths) in resisting arrows, and more effective than the siege rams in razing buildings, as it is one sturdy behemoth unit!
However, the slowness of the War Elephant is a major weakness that any player can exploit. They are very vulnerable to hit-and-run, but that can prove to be ineffective as the War Elephant is so strong. However, Saracen camels and mamelukes can destroy elephants for their bonus against other cavalry, especially the range of the Mameluke- the most damaging counter-unit of the War Elephant. War elephants can also be outnumbered by pikemen, and very easily. You need somewhere in between 3 pikeman to 1 elephant ratio, but the war elephants are so expensive that they are at a real loss here.
Another way to counter a war elephant army is to convert them with monks! Especially if you've got Block Printing (+3 conversion range) and Theocrazy (only one monk in the group needs to rest after a conversion). To add to that, the Persians lack Heresy, the research that kills the converted unit. This can become a problem, as your best weapon may become your worst nightmare! To avoid monk conversion, always have a squad of cavalry archers that can neatly support your war elephants, and can be used for a quick kill.
"Using elephants in battle was always a challenge because the animals have poor vision but keen smell, they are relatively slow, and don't like being injured. The Persians developed special training for the beasts and their riders, called Mahouts, to improve their battle performance."
Mahouts is the unique tech that gives the War Elephant a 30% speed boost. Although, it remains a pretty slow unit. The research is much needed, though, as it gives the war elephants that extra boost that makes them find their way into the enemy's town sooner. They become faster than rams, too!
Knights +2 attack vs. archers
A great bonus for Persian forces, as they also get fully upgraded paladins. Having +2 attack guarantees that the paladins will kill enemy archers faster, ensuring less damage to incoming arrows on war elephants and your other Persian units.
Any civ that uses the Knight line can benefit from this team bonus, so that is like saying all of 'em! The Saracens can only create knights, though, and their team bonus is useless with the Persians, unless they were going for an early Feudal rush to destroy farms. Anyway, a great combo of unstoppable rushing cavalry civs would be the team-up of Persians and Huns!
Persians would provide the team with archer-charging paladins, giving them more survivability against longbowmen, cavalry archers and hand cannoneers. On the other hand, the Huns would provide an awesome +20% stable working speed bonus to make those paladins and camels come out sooner!
The Persians' real strength is their cavalry
The Persians have one of the largest tech trees in the game, I think only third to the Byzantines and the Spanish really (correct me someone if I'm wrong). However, they lack the defenses to make them a powerhouse in the late game. Their lack of defensive structures and researches, makes the Persians primarily an attacking civilization.
The civilization with the perfect stable, having all cavalry units in the game, as well as every upgrade available to them! Paladins, heavy camels and hussars, in addition to the war elephants- the Persians have them all. The only and great weakness of this bunch is the Halberdier.
Great Archery Range
To counter infantry, the Persians have an almost perfect Archery Range, being able to create elite skirmishers, hand cannoneers and cavalry archers, as well as being able to research Parthian Tactics- the best upgrade for the Cavalry Archer!
Great Siege Workshops
Again, great support for the cavalry. The only real missing siege unit from the Siege Workshops tech tree is the Siege Onager. However, they can make do with heavy scorpions and bombard cannons!
The Persians are also a very proficient naval civilization. While not having a unique naval attribute, they can make fast fire ships, heavy demolition ships, galleons and elite cannon galleons. They are only missing Shipwright, but then again, their great economy compensates for the lack of this research.
Regarding economy-related researches, the Persians have all of them as well, meaning great economic power to aid the building of that expensive Persian army. Crop Rotation, from the Mill, is a particularly useful research for this civilization.
The Persian civilization is one of big powerhouses, but at the same time it can be fragile if not taken good care of! The bulk of the Persian army is cavalry, however, wins aren't made of only mounted units (it happens, though). For this, always have adjacent support to your cavalry, as their counters are cheap and can appear in large numbers.
The Aztecs are an option as a counter, because they can do Monastery researches to create sturdy monks. The Spanish are also a very early developer and can create large halberdier forces and send missionaries (mounted monks) to convert the Persian elephants! The Saracens are also an obvious counter for their cavalry-murdering mamelukes and powerful monks, too.
There is a great deal of fun when playing the Persians. They give you more liberty in the early game thanks to their resource bonus and capacity to gather them faster than most of the other civilizations. However, take too long to strike with your Persian forces, and your low defenses and cavalry won't be able to defend you.
Read my 10 of the Best Civilizations from Age of Empires II: The Conquerors for more info on more great civs!