Dr. Vellian Crowler with three "Ancient Gear Golems".
"Ancient Gear", known as "Antique Gear" (アンティーク・ギア Antīku Gia) in the OCG, is an archetype of EARTH Machine-Type monsters that are used by Vellian Crowler in Yu-Gi-Oh! GX; Rudolph Heitmann in Yu-Gi-Oh! 5D's; and Duel Academy students, such as the Obelisk Force members, Dennis Macfield, and Jean-Michel Roger in Yu-Gi-Oh! ARC-V.
They were released in The Lost Millennium, Shadow of Infinity, and Structure Deck: Machine Re-Volt, with further support in Light of Destruction and The Duelist Genesis. The archetype will also be supported by Structure Deck R: Machine Dragon Re-Volt.
They overwhelm the opponent while preventing them from activating Spell or Trap Cards when they attack, largely because Trap Cards and Quick-Play Spell Cards which activate during the Battle and Damage Steps tend to be some of the most common cards used by players, and the "Ancient Gear" archetype focuses upon preventing the opponent from using them. The concept of having limited Spell and Trap immunity is similar to the Duelist Kingdom arc of the second series anime, where Machine-Type monsters were immune to magical attacks. Due to the prevalence of monster effects that can be activated during the Battle Phase, the "Ancient Gear" introduced in Structure Deck R: Machine Dragon Re-Volt can prevent their activation too.
"Ancient Gear Gadjiltron" monsters share a relation with the "Gadget" archetype, gaining additional effects when "Red", "Green" and/or "Yellow Gadget" are Tributed to Summon them. This relationship is further extended with their own Gadget monster in "Ancient Gear Gadget".
The humanoid "Ancient Gears" bear a striking similarity to Ancient Greek soldiers, and so their creation may be linked to the Antikythera mechanism.
True to their name, all "Ancient Gear" monsters resemble armored robots of varying designs with their bodies featuring unions of cogwheels and gears, joining several parts of their bodies together.
Their name "Ancient Gear" is likely a reference to the old age of the mechanism that governs their functioning, which normally would require an external force to move them but interestingly enough they seem to be self-sufficient. This implies that contrary to their apparent antiquity, that they may be far more advanced than they look.
When they are used as Fusion Material by members from Duel Academy (in Arc-V) their Summon Chant mentions that they carry the "ancient spirits" or other similar terms. This seems to imply that the "Ancient Gear" monsters may be old machines possessed by spirits, which could explain their functioning despite their old workings. This also seems to be the reason behind their glowing eyes.
Most "Ancient Gear" monster possess an effect that prevent the opponent from activating any Spell/Trap Cards until the end of the Damage Step when they attack. This allows "Ancient Gear" monsters to attack without fear of being stopped by any Spell/Trap Cards. However, "Ancient Gear" monsters do not stop Monster Effects. A number of cards allow "Ancient Gear" monsters to be Summoned from the Deck, such as "Spell Gear" and "Machine Duplication". Most "Ancient Gear" monsters possesses high Levels so they require Tributes to Summon. Cards such as "Geartown" and "Ancient Gear Castle" allows ease of Tribute Summoning "Ancient Gear" monsters. "Cyber Dragon" can also give some ease of Tribute Summoning "Ancient Gear" monsters. "Ancient Gear Workshop" is also recommended if you use "Ancient Gear Golem" and "Ancient Gear Beast", as they cannot be Special Summoned.
"Ancient Gear Drill" can be used to search out some Quick-Play Spell Cards like "My Body as a Shield", "Shrink", and "Limiter Removal". These cards work well with "Ancient Gear Drill" as they can be activated outside your own turn. Cards like "Ancient Gear Castle" and "Geartown" to make Normal Summoning the Level 5 or higher "Ancient Gear" monsters easier.
"Ancient Gear Gadjiltron Chimera" and "Ancient Gear Gadjiltron Dragon" can be used in combination with the "Gadget" monsters to unlock power effects. Since all of the "Ancient Gear" monsters are Machine-Type, the Spell Cards "Limiter Removal" and "Solidarity" can be necessary.
"Ancient Gear" monsters are not immune to Trap Cards (aside from targeting "Ancient Gear Engineer"). If your opponent has a Set "Mirror Force", for example, attacking with a non-"Ancient Gear" monster will allow your opponent to destroy all of your Attack Position monsters. Another vulnerability to look out for are Monster destruction/banishing effects.
Cards like "Acid Rain", "Magnetic Mosquito", and "System Down" have the potential to shut down an "Ancient Gear" Deck completely. Although these cards can be hard to counter, "Imperial Iron Wall", "Starlight Road", "The Huge Revolution is Over" and other Spell/Trap negation cards can prevent them activating. Again, "Ancient Gear Beast" can negate Flip Effect Monsters such as "Magnetic Mosquito".
Finally, being Machine Monsters, they are especially vulnerable to "Chimeratech Fortress Dragon", which can allow your opponent to clear your field and gain a 2000+ ATK beatstick. For this reason, you should never leave a "Cyber Dragon" on your side of the field whilst you control another machine. It may also be wise to add "Battle Faders" to your deck to prevent your opponent from attacking directly, as well as giving you tribute fodder for the higher level Ancient Gears. Combine this with cards that change battle positions such as "Enemy Controller" or reduces ATK such as "Shrink" and you could easily turn the tables to your favor.