Trash units are units that cost no gold and are usually meant to overwhelm your opponent with quantity rather than quality. They’re generally not great fighters (except against the units that they are supposed to counter). Trash units are good as both a meat shield and also, as the main units when you’re out of gold.This generally happens in 1v1 games when all the gold on the map has been mined and relics are too slow to generate gold.

The main trash units are:
  1. Skirmisher/Elite Skirmisher
    These are created at the archery range and are generally good to counter archer type units which includes hand cannoneers.
    They also counter the Spearman line well when massed.

Blacksmith attack upgrades applicable: Fletching, Bodkin arrow, Bracer
Blacksmith armor upgrades applicable : Padded Archer Armor, Leather Archer Armor, Ring Archer Armor
Other regular upgrades it benefits from: Thumbring, Conscription, Ballistics.
Unique technologies it benefits from: Yeomen, Atlatl, Andean Sling, Imperial Skirmisher (When teamed with Vietnamese)

Civilizations with good bonuses/Technologies for them: Britons, Vietnamese, Aztecs, Magyars, Byzantines, Spanish

skirmisher 300x148 - Trash Units in Age Of Empires II

2. Spearman/Pikeman/Halberdier
These are created at the barracks and are used to counter cavalry. When fully upgraded, halberdiers are a great unit capable of countering cavalry big time.
They’re almost as fast as the cavalry they fight. It costs just 25 wood, 35 food

Blacksmith attack upgrades applicable: Forging, Iron Casting, Blast furnace
Blacksmith armor upgrades applicable : Scale mail Armor, Chain mail Armor, Plate mail Armor
Other regular upgrades it benefits from: Tracking, Squires, Arson
Unique technologies it benefits from: Garland Wars, Druzhina, Chieftains, Perfusion

Civilizations with good bonuses/Technologies for them: Goths, Vikings, Aztecs, Japanese, Slavs, Malians, Ethiopians, Spanish, Celts, Burmese

spearman 300x144 - Trash Units in Age Of Empires II

3. Scout cavalry/Light cavalry/Hussar
These are created at the stable and are primarily used to counter monks. However, they do pretty well against archers (except maybe the Genoese crossbowman) and siege as well
They cost just 80 food.

Blacksmith attack upgrades applicable: Forging, Iron Casting, Blast furnace
Blacksmith armor upgrades applicable : Scale Barding Armor, Chain Barding Armor, Plate Barding Armor
Other regular upgrades it benefits from: Bloodlines, Husbandry
Unique technologies it benefits from: Chivalry

Civilizations with good bonuses/Technologies for them: Turks, Magyars, Berbers, Mongols, Franks, Spanish, Huns(team)

scout 300x145 - Trash Units in Age Of Empires II

Other Trash units:

1. Genitour/Elite Genitour (only when teamed up with Berbers)
These are created at the archery range and are available to those teamed up with the Berbers. They’re mounted skirmishers and so, counter archers quite well. If the Turks team up with the Berbers, they actually get a buffed mounted skirmisher (which benefits from their unique tech). They cost 50 wood, 35 food.

Blacksmith attack upgrades applicable: Fletching, Bodkin arrow, Bracer
Blacksmith armor upgrades applicable : Padded Archer Armor, Leather Archer Armor, Ring Archer Armor
Other regular upgrades it benefits from: Thumbring, Conscription, Ballistics, Bloodlines, Husbandry

Unique technologies it benefits from: Sipahi

Civilizations with good bonuses/Technologies for them: Britons (team bonus), Vietnamese,Magyars, Byzantines, Spanish, Turks, Huns, Mongols, Saracens

genitour 300x169 - Trash Units in Age Of Empires II

2. Magyar Huszar (Magyars only)
These are created at the castle and are buffed version of the scout line. They do not receive an anti-monk bonus but they perform all other tasks better than a regular hussar. They get an attack bonus vs. Siege and are available only to the Magyars. They become a trash unit only after Mercenaries tech is researched.
They cost just 80 food.

Blacksmith attack upgrades applicable: Forging, Iron Casting, Blast furnace
Blacksmith armor upgrades applicable : Scale Barding Armor, Chain Barding Armor, Plate Barding Armor
Other regular upgrades it benefits from: Bloodlines, Husbandry
Unique technologies it benefits from: Mercenaries

General upgrades/bonuses all units benefit from: Faith, Heresy, Teutonic conversion resistance team bonus.

magyar huszar 300x169 - Trash Units in Age Of Empires II

3. Imperial Skirmisher
Available to the Vietnamese and their allies. It’s an upgraded version of the Elite Skirmisher. It gets an extra pierce armor and an attack point. It’s the first unit from the right in the skirmisher picture.

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Generally top tier civilizations for trash units: Magyars, Spanish, Britons, Goths, Japanese, Byzantines

Turks have the worst trash units in the game as they neither get elite skirmisher nor pikeman (let alone halberdier). So, once all the gold runs out, they are fucked.