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Dragonmech is a d20 setting that mixes fantasy and steampunk technology from Goodman Games. It introduces rules for more advanced weapons, and large, lumbering piloted walkers called mechs.
The world of Highpoint was a slightly less advanced fantasy world where
- Most humans were nomads on the Endless Plains
- Elves practiced magic in their forest homeland
- Dwarves were in their underground homelands
- There were few cities
...until a hundred years ago. At that time, the moon, for unknown reasons, shifted closer to the planet. The planet's gravity began to strip off the lunar surface as the moon passed over, causing a nightly 'lunar rain' that can be as mild as burning, abrasive dust storms, to as violent as full-blown meteor rains. This caused a panic on the entire continent of Highpoint--which doubled with companion discovery; The moon turned out to be home to powerful creatures, some of which are robust enough to survive the trip from the planet to the moon. Most powerful of all were the lunar dragons, which are larger and even more terrible than the metallic and chromatic varieties native to Highpoint.
There was a tragic period where the populous humans attempted to rush into the underground lands of the dwarves and other peoples who lived below ground. The dwarves lost several holdings to the inrush of panicked refugees, and then huge increase of population pressure was felt even into the Underdark and below.
After some years of this, an aged dwarf appeared in the dwarven hold of Duerok, and persuaded people that he knew a way to retake the surface from the huge lunar monsters that were terrorizing it. He instructed the craftsman in building specific parts, and with his supervision, the city built the first modern mech. The skepticism of the people was erased completely when a young, dwarf trained to operate the mech slew a lunar dragon--though he died in the process.
The old dwarf claimed he was from a Gearwright's Guild, which held records of machinery going back to the 'First Age of Walkers,' which was so long ago that not even the elves have records of it. (The elves are skeptical that this Age ever happened). Nonetheless, the dwarf Parilus really did know about machinery, and he left his three junior Gearwright assistants to teach people in the uses of technology, clockwork, and steam.
In the near-century since then, the dwarves have constructed 5 'city-mechs'--so large they can be a home to thousands--which control a 'mechdom', the humans have united behind a charismatic leader for the first time and have built two city-mechs of their own. The elves have developed their own school of mech design--based on magic-powered constructs--and built the most powerful city-mech in existance (a full half-mile high), and there's an entire mech-centered culture that has the best known pilots.
Dragonmech uses the core d20 classes with some guidelines (for example, sorcerors are more common thanks to lunar influence, lack of formal armies makes barbarians more common than fighters, there are more Paladins, but far fewer druids), and also introduces base classes specific to the setting--Mech Jockey (hotshot mech pilot), Coglayer (gadgeteer), Steamborg (steam-powered cyborg), Constructor (variant Wizard), Stalker (variant, mech-focused Rogue), and Clockwork Ranger (variant Ranger who lives in the inner workings of the huge city-mechs).
It also details the rules for mech construction and use, as well as several kinds of mechs. An important fact to bear in mind is that Dragonmech mechs are large, clumsy, lumbering, noisy affairs which need large crew requirements. They are not like anime-style mecha, such as might be found in Gundam Wing.
 Clockwork Ranger
The clockwork ranger protects the 'gear forests' of the huge engines of the city-mechs. In an environment full of gears spinning like buzzsaws and pumping pistons, they thrive by understanding the hazards and the locals. And, indeed, the Gear Forests are developing their own ecology--such as the alligator-like Grease Lizards.
Coglayers are, essentially, the gadgeteers who build the intricate steam- and clockwork-powered devices that now populate Highpoint. They are unsuited for melee combat, with poor BAB and fragile hit points. They contribute by having a huge number of skill points, every technology-related skill (even the Pilot Mech skill), and the ability to create devices using 'steam powers,' which can be made at a small cost for each of them, and then combined into weapons, utility devices, or enhancements. In particular, their low BAB is negated by their ability to make weapons which inflict high damage on a touch attack.
The Constructor is a wizard who specializes in building constructs. They have their own spell list, and are considered to be specialist wizards. Furthermore, all Constructors belong to the College of Constructors, which sets a few light restrictions upon its members.
 Mech Jockey
The star mech pilots, mech jockeys have a talent for pushing mechs to levels of performance even the mech-builders were unaware of. And while they are physically unimpressive on their own, they can be powerful tanks/assault characters when put into a mech, actually having a personal BAB (medium) and a Mech BAB (Full)
The Stalker is a variant Rogue, with only a few differences. First, the Stalker loses a few skills, such as Forgery, and in their place gains skills related to mechs and technology. They also gain specific special abilities that help them with their task of mech infiltration.
The steamborg is a character class centered around replacing the character's body with more powerful, steam-powered prosthetics. At the first level, they are implanted with a specialized steam engine. At every even-numbered level, they gain an "Artificial Parts bonus," which may be used to increase Strength or Dexterity, or even Natural Armor and Attack Rolls. This comes at a price, though--the steam engine drains the Steamborg's body of water to give itself steam, making the steamborg require roughly threee times the hydration of a normal person, or lose function of their steam-powered parts until they again hydrated. Even more insidious, the steamborg has a chance of waking up and thinking and acting in an mechanical, ultra-logical way, each day.