By Rapscallion
Rise of Nations offers many opportunities to use certain units to counter other types of units. Nowhere is this more blatant than the stables.
The heavy cavalry and horse archer lines have their own specific uses, but the light cavalry line is often underused. The trick to using the light cavalry is to realise that they are a counter line with no main combat role other than that, but they are very good in this regard.
Before considering using any of the light cavalry line, bear these following three factors in mind.
First, light cavalry are cheap - not the cheapest combat unit by far, but they are the only early cavalry that doesn't cost gold. Instead, they require food and wood, which are often the most affordable resources around.
Second, light cavalry are the fastest early unit around. Up until and including Enlightenment age, the light cavalry line is the fastest non-unique troop type around. The heavy cavalry line starts at the same speed as the horse archer line - 30 - and they increase one speed per age until Enlightenment, but the horse archer line remains at speed 30 until they become scout cars in the Industrial age. The light cavalry line start at 41 and remains there, but it is always a third faster than traditional raiders, and they always have a decent advantage over the heavy cavalry line. They are the only early cavalry unit fast enough to catch a retreating scout or general.
Third, the light cavalry line is one of the lightest troop types around. They cannot take much punishment. Though they are fast, they often fall to passing strikes.
The standard use of light cavalry is to chase down enemy horse archer raiders. Light cavalry are faster than horse archers, and their inherent bonuses can allow them to combat horse archers even when outnumbered by a factor of two to one. A common defence to raiding is to place a light cavalry unit where you really don't want to be raided. I would suggest only having one light cavalry unit against one horse archer unit, as chasing cavalry units often get in each other's way.
Light cavalry are also popular for using in the 150 attack. The ancient rush relies on a slinger to counter the defender's archers, but in the Classical age, the javelineer is not up to the job, and the light cavalry line is often used instead. One light cavalry unit will easily kill one archer or slinger, so take two or three, or more if possible, but operate them to the side of the main conflict for easy micromanagement.
Heavy cavalry are often best used to one side of a battle to charge in on the flanks of a fight for the bonuses there against occupied troops. Light cavalry, however, are fast enough to run around a battle and impact from the rear. Send them around a city or a mountain to do this - most battles last long enough to allow light cavalry to manoeuvre this way. The bonus for impacting on an enemy's rear is not as good as that of the flank, but the type of units available make for juicy targets. The enemy patriot and generals are good targets, as are scouts - allowing your spies to do their work - supply wagons, siege, and so forth. Beyond these are the light infantry ranks in support of the heavy infantry, and this is where your bonuses apply.
Of course, this is a textbook attack, and your opponent will often have reinforcements coming up to supplement their attack, but light cavalry are cheap, and the units you often see at the rear of an army are not.
Care must be taken when using the speed of light cavalry to support an ally in a team game. Yes, light cavalry can get to their destination faster than any other troop type, but what will they do there? Always take a few seconds to evaluate what you can see of an enemy's position and troop types. If you can guarantee hitting his army in the rear, or if his troops are all light infantry, go for it - but only when you have a few cavalry at your disposal. If you cannot, use other troops.
Light cavalry will lose out to heavy cavalry in the early ages, as is right and proper, but the margin of loss isn't actually that bad. In each age up to enlightenment, one light cavalry of the age will leave one enemy heavy cavalry with just over half its hit points remaining. Two light cavalry against a heavy cavalry will usually leave one slightly damaged light cavalry remaining. This is useful to know for cases when you cannot afford the metal for heavy infantry or heavy cavalry - light cavalry are cheaper than heavy cavalry, but the trade-off comes into play when you consider the resources involved. The food and wood required for light cavalry is usually easier to obtain than metal or gold.
The other consideration here is that you may need to find some of the most precious resource - time. Light cavalry can delay an enemy force, particularly heavy cavalry, for a relatively low cost. Taking this into the latter ages, hussars are the best flesh-and-blood light cavalry available to most nations. I have used them effectively to slow small numbers of light tanks down for low cost, even destroying some in the process. Using the light cavalry line for this, though, should really only be used in small-scale confrontations, such as when defending raiding.
Unique units allow for the occasional twist. The Russian cossack line is cheaper than ordinary light cavalry, but it also has a bonus against siege weapons and supply wagons. This makes such enemy units into prime targets for such cavalry troops.
The Egyptian light camel line, however, gains bonuses against other light cavalry. At first, this may not seem to be too useful, but consider the early classical raid strategy. The best defence against a raiding horse archer is to send a light cavalry unit after it. I normally try to screen my raiders with heavy cavalry, but the Egyptian light cavalry units would do the same job for less resources, and they have a better speed.
The light cavalry line is fast, and it has a range of units it counters neatly. However, there are ways that you should not use them. These are the abuses.
Heavy infantry of any sort will carve through light cavalry in a straight fight, barely pausing to gather their breath. Even having the cavalry run past them will result in the loss of units from passing blows. Gunpowder age and beyond often involves light cavalry perishing at a range from gunpowder-armed heavy infantry, though they are still useful against the arquebusier line, including Chinese fire lances.
Some people have tried to use light cavalry for raiding, but this is generally ineffectual. They do not cause enough damage to citizens to kill them quickly, and militia will see them off soon enough. Similarly, they don't damage merchants or traders enough to kill them before defenders come to the rescue. Occasional unique units, such as the Russian Cossack line, can do this on occasion, but the horse archer line is usually far better at this task. At best, when raiding and using light cavalry, use them as a screen to defend your raiders.
Never consider any of the light cavalry line to be anything other than a counter unit or a delaying force. Even used as a delaying unit, they are not very effective. Use them in targeted harassment, but never as a main combat force. Try to use them with finesse, rather than charging in, but always expect to lose them. The light cavalry line can deal targeted damage against certain units effectively, but it is rare that they will survive for long. The best use is to operate one or two packs of five or six, and target them at specific units in an enemy army.
Light cavalry have their limits, but never neglect them.