Age of Empires Series Wiki
Age of Empires Series Wiki

Armored breastplate increases hitpoints but reduces speed of heavy infantry, such as Musketeers, Rodeleros and Pikemen (among others).
—In-game description
Rodelero and Pikemen hitpoints increased, but speed reduced.
—Knights of St. John in-game description

Corselet[sic] is a technology in Age of Empires III that is unique to the Spanish and Knights of St. John, although the latter only have access to it in Act I: Blood, and can be researched at the Church once the Royal Decree to Claim the New World Home City Card has been sent. Once researched, it increases the hit points of all heavy infantry by 25% but reduces their speed by 15%.