Gareth is a special unit in Stronghold Legends and only available to those users who are currently playing as the Arthur faction. He is recruited from The Round Table, and costs 500 gold and 100 Honour. You can find out ways to generate Honour in the game here.
His general skills and abilities mirror that very much of a standard Knight. His attack strength is virtually identical to that of a Knight in standard combat as well- in straight out combat between Gareth and a Knight, it's essentially the unit that strikes the first blow that will win.
By default, Gareth will be mounted when you purchase him from The Round Table. You can dismount him from his horse by selecting him and then clicking on the "dismount" icon.
Once mounted on a horse, any enemy units must first kill the horse before they can attack Gareth himself, so there are great advantages of doing so, besides Gareth moving a lot faster. The horse alone takes 2 Crossbow bolts to kill.
If the Horse is killed or you dismount Gareth, you can mount Gareth on another horse, but must wait for a short "recharge" time before this button becomes available again. To do this, select Gareth and then click on the "Mount horse" icon. Note that by mounting Gareth, like any unit, it will cost 20 Honour. Once you have attempted to mount him, a horse will instantly appear and he will automatically be mounted.
You can only have one Gareth unit at any time. If he is killed, you will instantly be able to purchase another Gareth unit from The Round Table again. Like all ground units from The Round Table, Gareth has a special power, in his case called The Eye of Truth. This special power will uncover any enemy man traps within the vicinity and destroy them. Note that Gareth must be close enough to the area you wish to search for traps in order to be able to use this power.
Some traps in the outer area of the vicinity you checked may be revealed but not destroyed. You can use this power again to destroy those traps, but since they have already been uncovered they will not trigger unless your troops walk over them. Note that this power will work on both Killing Pits (Ice faction) and man traps (Evil faction).
You can activate this special power by left-clicking on the red eye icon on the right side of your screen, and right clicking on the map where to use it. It can only be used somewhere within Gareth's range already, and once used, it will require a "recharge" time. During this, the green line around the outside of the red eye icon on your screen must reach all the way around to the start before it can be used again.
This icon is only available while Gareth is alive, so if Gareth dies or you have not yet recruited him, you must recruit another Gareth from The Round Table again in order to use it. Only he has the ability to use this power.
However as Gareth is so similar to a Knight, the same weaknesses also apply to him. Without use of his horse, he is a very slow moving unit. He can easily get overwhelmed by defence systems such as Burning Logs, Ballistae or pitch ditches. And he can also very easily be picked off by ranged or aerial units, frozen by Ice units and have his health sputtered away over time by persistent attacks or fire. This is primarily due to the fact he moves so slowly.
Gareth will also struggle against units such as Frost Giants, even whilst mounted, simply because Frost Giants are so powerful and can cause an enormous amount of damage. It's therefore extremely important to protect him well to prevent this from happening, particularly if the enemy is playing as a different faction with access to more advanced units and magical powers, as most of these are not limited.
Tip: Gareth's power is very effective at clearing your path of enemy defence systems which could harm your units. If you are under any doubt at all about whether there could be any man traps or killing pits in front of you, it's far safer to use his special ability to check if there are any traps.
At worst, you've used the ability for a short time and know that the path is clear and safe for your units to advance down. If you've just created a breach using Bedivere's Horn of Camelot or some other way, you may wish to search inside an enemy castle to make sure there are no traps before sending your troops in to attack.