Laddermen are the simplest form of siege equipment available, available to all factions, and you can recruit them from the Siege Camp.
Unlike in Stronghold 2, you do not need to place an Engineer's Guild before you can recruit units from a Siege Camp.
Laddermen will run towards the enemy walls and place a ladder on the outside to allow friendly troops to climb up and gain access to the enemy castle.
Once a ladder has been placed, Laddermen will run back to the Siege Camp and fetch another ladder to try again. They will continue doing this until eventually killed.
If no Siege Camp is placed or it has since been destroyed, the Ladderman will do nothing, simply staying at the bottom of the ladder once it is placed on the wall.
You can destroy enemy Ladders on your own walls by having troops push down the ladders from the top of the wall, destroy the ladder from the ground, or you could also use other means to destroy the ladder, for example by erasing the wall and constructing it again.
Laddermen are virtually useless in melee combat and have an attack strength even worse than the weakest military units. They have absolutely no attack strength whatsoever while holding the ladder and cannot defend themselves in any way, even if they are being attacked by a melee troop.
Similarly, Laddermen can technically be ordered to attack units, but they will simply stand there and take damage instead of causing any damage to the enemy. The Ladderman can be picked off by ranged units virtualy straight away as well, because they have no protective armour - Archers take just 2 shots and Men-at-Arms take only a single throw of their Javelin.
You will need 4 gold as well as 1 peasant from the campfire before you can recruit a Ladderman.