Men-at-Arms are part of the standard troops available to all factions in Stronghold Legends, and are the traditional spear unit available in previous games of the series. They replace Spearmen from Stronghold 2, and perform much the same function, although in Stronghold Legends Men-at-Arms have changed quite a bit.
To begin with, Men-at-Arms move very fast- they're virtually one of the fastest units in the entire game. This provides you with great maneuverability when using your troops, especially if you intend to setup some kind of ambush.
When an enemy unit appears within range, the Man-at-Arms will throw their Spear at the enemy unit, often killing them outright if they are unarmoured. Even if the unit is not killed straight away, a strong amount of damage is dealt by your Man-at-Arms, who will then run in with a Sword to fight the unit in a melee styled attack. Men-at-Arms are poor in melee attacks, but in enough numbers can take do a lot of sustained damage in melee attacks.
Using their ranged attack, large numbers of Men-at-Arms can be lethal against even the most heavily armoured units. The Spears they all throw will deal out serious damage against enemy units and unless there are enough enemy units to even out the damage or overpower them, you can quickly lose a lot of units. Men-at-Arms are particularly useful against Archers, as their Spears can kill Archers in a single throw. Once thrown, their Spear takes a short 'recharge' time before it becomes available for them to use again.
Crossbowmen, who use more powerful bolts than Archers as well as Leather Armour, can kill Men-at-Arms in a single shot due to their lack of Armour. Men-at-Arms require a spear from a Poleturner's Workshop to be produced and stored in the Armoury before they can be recruited. You will also need a single recruit from the campfire before you can recruit a Spearman.
Keep in mind that while Men-at-Arms are technically ranged units, they cannot counter aerial attack in any way, nor can they launch one. If your Men-at-Arms are suddenly attacked by Dragons, White Witches or any other units from the sky, or magical powers, there is little if anything you can do to stop this unless there are units or defensive structures stationed nearby who can counter these attacks on their behalf.
Men-at-Arms are fast, mobile, cheap to produce and do not require any Honour. For this reason, they are the ideal troop to mass produce, especially earlier during a game when you have less resources available.
Note that Men-at-Arms are vulnerable to Vampiric Creeper attacks. If you are playing against a player who is currently the 'Evil' faction, you must ensure you take adequate measures to protect your Men-at-Arms against these attacks or you could risk losing large swathes of your troops to the enemy. Vampiric Creepers can also climb up walls and attack your troops stationed on gatehouses or towers as well, so it's important you keep this in mind.
Tip: As with most weaker units, Men-at-Arms are best served in stronger numbers. Given the right support by ranged units, there are virtually no units (on the ground) that Men-at-Arms cannot kill. Use Men-at-Arms wisely to make sure that your regular and more powerful troops take less damage, but in numbers they can be quite successful troops.