Ironclad is a technology unique to the Teutons that can be researched at the Castle in the Castle Age in Age of Empires II: The Forgotten.
This technology gives Trebuchets, Rams, Scorpions, Mangonels/Onagers and Bombard Cannons +4 melee armor. Players must keep mind that while Rams' displayed melee armor value is 0, it's actually -3, so researching this tech gives Rams a total melee armor of 1.
Strategy
Siege engines are normally very weak against melee units due to a number of factors. First, all but Rams have a minimum range, while Rams deal little to no damage to anything not a building, and Scorpions are still vulnerable to most Cavalry units unless massed. Secondly, siege units are amongst the slowest in the game, and can't separate themselves even from Infantry units if they get up close, because of these disadvantages even a single villager can deal with single siege units. This technology improves the survivability of siege units by giving them melee armor, which is very helpful if they get separated from the player's main force or focused down by it's opponent. The benefits from this tech are higher than full Blacksmith armor upgrades against melee, and cancels out the bonus damage from the upgrades found in the same building.
The player must keep in mind, the bonus armor by no means lets their siege units act out on their own. It gives to the player some time to react if they get targeted and protects them from single units, but they should still be backed up by an army, preferably on the back of it.