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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%.

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Unique religious technologies in Age of Empires III
AOE3Icon.png Age of Empires III
Flag BritishDE.png BritishThin Red Line · The Black Watch · Roger's Rangers
Flag DutchDE.png DutchCoffee Trade · Waardgelders · Blue Guards
Flag FrenchDE.png FrenchCode Napoleon · Imperial Young Guard · Imperial Middle Guard · Imperial Old Guard
Flag GermanDE.png GermansTilly's Discipline · Wallenstein's Contracts · Zweihander
Flag John Black's Mercenaries DE.png John Black's MercenariesIndian Friendship · Customized Merc Weapons
Flag Maltese Cross.png Knights of St. JohnCorselet · Tax Burden · High Crusade
Flag OttomanDE.png OttomansNew Order Infantry* · Tufanci Corps · Topcu Corps
Flag PortugueseDE.png PortugueseEncomienda Manor · Besteiros · Order of the Tower and the Sword
Flag RussianDE.png RussiansWesternization · Kalmucks · Petrine Reforms · Bashkir Ponies
Flag SpanishDE.png SpanishCorselet · Royal Alabarderos · Wild Geese
Flag American act3 aoe3de.png United States (Act III)Grand Encampment · American Patriots
Age3DE Icon.png Definitive Edition
Flag SwedishDE.png SwedesPush of Pike · Savolax Jaegers · Gustavian Guards
Flag American act3 aoe3de.png United StatesPennsylvania Pound · 1st Pennsylvania Rifles · Pennsylvania Cavalry
Flag MexicanDE.png MexicansFirst Guarantee · Second Guarantee · Third Guarantee
Knights of the Mediterranean
Flag ItalianDE.png ItaliansCarabinieri · Risorgimento · Papacy
Flag MalteseDE.png MalteseTeutonic Knights · Crusader Knights · Knights of the Hospital
All of the listed technologies require the "Royal Decree" Home City Cards unique for each civilization, except the random map United States ones which require the "Philadelphia Convention" Federal Card