How to play Horseman
The Horseman is a very powerful unit. It can scout for villages and pick off lone units, but it can also form your front line of combat or hang back and kill wounded units in the middle of a battle. It has high HP, a good attack, many moves, and resists most damage types. The one downside: It costs a lot.
Hopefully, this guide will make the Horseman worth its cost to you, or even worth more than that.
When to get the Horseman
The Horseman has a very different role in campaigns, as opposed to multiplayer. In campaigns, the goal is usually to win quickly, which the Horseman excels at, while in multiplayer, the goal is to win at all, which entails many different strategies.
There are very few situations in a campaign in which Horsemen would not benefit you. Early on, Horsemen can win you some quick victories and gold; later, a corps of Knights or Grand Knights is perfect for doing the real heavy work of a battle. The Grand Knight is one of the highest-HP units in the game, so it makes a very good line-holder in an open area without normal defensive terrain, which would otherwise be a weakness.
The Knight really isn't a unit that you can do without, if you're allowed to get it. Knights are the perfect shock troops to use where other units fail you. In Heir to the Throne, Horsemen and Knights can win quickly in a battle which would otherwise involve slogging through streams of enemies, such as Crossroads or Northern Winter; in The Rise of Wesnoth, many scenarios, like the Midlands have open ground, easy for horse units to dominate, and lots of Orcs, often the ideal opponents of Knights; in the Eastern Invasion, Knights can be made into Paladins, perfect for fighting the Undead, or Grand Knights, which massacre everything else in the campaign, and can be given holy ankhs to make them even more devastating than Paladins.
It's usually not such a good idea, though, to use Horsemen in cave scenarios. The exception is when you have a Paladin and face Undead; if the map is small enough that you can get it to the battle, its holy weaponry makes up for its other weaknesses.
The Horseman is significantly less valuable here. In campaigns, you often have more gold than you could possibly spend, making cost no issue; in multiplayer, cost is always an issue.
For this reason, in multiplayer, you cannot get hordes of Horsemen, and deal devastating damage to all the enemies. However, you can still get a few horsemen and deal devastating damage to a few enemies.
If the enemy fields too many low-melee archers, get a horseman. If the enemy fields too many Drakes, get a horseman, If the enemy fields too many Spearmen, don't get a horseman! Overall, in the Default era, the Horseman is best against Northerners, mediocre against Rebels, Drakes, and Knalgans, and downright bad against Loyalists and Undead.
Never get Horsemen against Loyalists. Loyalists regularly use Spearmen, Bowmen, and Heavy Infantry, all of which are devastating to Horsemen. As a scout, the Cavalryman is cheaper and less likely to get massacred.
Don't get Horsemen against Undead either, unless the Undead player has already recruited stupidly (e.g. all Dark Adepts.) Even in that situation, Cavalrymen may be the better choice.
None of the Knalgans' units is that great against the Horseman, but only the Poacher is weak to the Horseman, and it's best not to field Horsemen unless they can be sure of some effectiveness.
Elves are frail and easy to take down with a Horseman, but if you let it near them, an Elvish Shaman would be very nasty, and the Horseman is no good at all against Woses. (Side note: A Knight is very effective against Woses.) If he goes with Shamans and Woses, don't get a horseman. If the opponent uses Elvish Fighters and Archers a lot, do get a horseman. Just don't be stupid and attack them in forest.
Drakes are another dubious case. If the opponent gets Clashers or Saurian Skirmishers, then the Horseman is very vulnerable and shouldn't be recruited; however, if they don't, then a Horseman will easily cut down Burners, Augurs, and even Fighters. On the other hand, it's a pretty weak opponent who will fail to get either Clashers or Skirmishers when faced with a Loyalist opponent.
Against Northerners, a Horseman is almost always effective. All northerner units are Chaotic, meaning that you can get a damage bonus while your opponents have a damage penalty. No northerner unit except the Goblin Spearman is any challenge to the properly played Horseman, and the Spearman, though it is a risky fight, is so weak that it can often be killed in one hit.
What do do with the Horseman
Attack the following: Low-melee archers, low-defense one-hit-kills, and units that need to die ASAP to win the battle. Do not attack the following: High-melee units, high-defense full-HP units, and units that can kill the Horseman and thereby lose you the battle.
The Horseman is an opportunist. If there's something to attack, do so. If there's nothing to attack, run away and grab a village.
A common mistake with the Horseman is to attack, kill something easily, and then have the Horseman cut to ribbons the next turn by archers. This is no good. The Horseman costed more than that unit. One thing never to do is to attack with the Horseman when it is open to attack like this. Always defend the Horseman if it attacks from a position open to enemy units.
Defense isn't always that obvious, though. If the Horseman can attack, be killed by archers, and then your army can kill the archers, then the Horseman is defended. If the Horseman can attack, be killed by archers, and then you can break through their battle lines which are no longer supported by the archers, then the Horseman is defended. If the Horseman can attack, be killed by archers, and then you can grab enemy villages and split the enemy army, then the Horseman is defended.
Of course, you can always defend it by putting units around it, but be careful that you won't lose the battle just because the enemy kills a guard and then sneaks in and shoots down the Horseman. Threats are more secure, if the enemy can't remove them.
Another common mistake with the Horseman is to attack a high-defense fighter that can kill the Horseman, just because the Horseman can kill it in one hit. Now, if killing that fighter is crucial to your victory in the battle, then by all means, do it; but if the fighter is in an insignificant side position, then the attack is suicidal. Remember, a wounded unit is probably worth about half as much as the Horseman. If you let it do damage to you, it had better be very little damage, or very unlikely, because otherwise, you're losing out.
Also, never attack a unit that can kill the Horseman and level up by doing so, unless it is the only way to stop it, even if that unit is in 20% defense and can be killed in one hit.
A third mistake, just as bad as the others, is to leave the Horseman running around grabbing villages and never reaching the main battle. It's a waste of gold. You can save five gold on that by using Cavalrymen instead (or Elvish Scouts if this is HttT) but even then, it's pretty wasteful. The unit has an attack and is meant to use it.
At the very least, if there are truly no attack opportunities, keep the horseman close enough to the battle to strike at any moment. The threat cripples your opponent's strategy, especially relating to his leader. The leader is often a level 2 unit which can be taken out in one attack by a lucky Horseman; if you see an opportunity to swing for the enemy leader, (although not so much one of several enemy leaders in team games) you should usually take it. It gives you a chance to win the game immediately, and even the worst-case situation, losing the Horseman and doing no damage, doesn't decrease your overall chance to win by that much.
Finally, remember the time of day. You don't have to worry about it too much in itself, as long as you check how much damage you're actually doing, but you'll find that the horseman is much more effective during the day, obviously - you may want to retreat your other units at night, or not commit to a battle that will last until night, if you want to rely on Horseman support.
You choose, when leveling up, between the Knight and the Lancer.
In campaigns, always get the Knight. The Lancer is easier to kill and doesn't advance, which are important in campaigns, while a few extra moves and more raw damage are not. In campaigns, you can't afford to lose random level 2 units, and the Lancer really can't hold the line either.
In multiplayer, a lancer may be a decent alternative choice. They have more strikes with their charging weapon, giving them more damage potential overall. The huge movement they have also allows them to get through enemy defenses and hit a leader.
In Heir to the Throne, or The Rise of Wesnoth, where gold is plentiful, a prime strategy is to recruit castle after castle of Horsemen and run down the enemy by sheer numbers. Low-XP horsemen can be used for suicide rushes on powerful units, while high-XP horsemen can be used to take out weak units, such as one hit kills or Archers and Troll Whelps during the day. If you are going for level 3 knights, then many units can be killed in one hit by a Knight's charge, but if you want more level 2 knights, their swords are also very useful for softening up enemies for Horsemen to kill.
How to play Loyalists article
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