Alien World ruleset is now part of Freeciv project. There has not yet been any releases of it as part of Freeciv, nor will be in near future, but it's part of Freeciv development. Most likely it will be part of main Freeciv package in Freeciv-3.0. Until then it's available as separate modpack.
Marko Lindqvist (cazfi) started development of Alien World as custom ruleset 2008. Its first public release was compatible with freeciv-2.3. Ruleset was added under freeciv version control 2012, when freeciv TRUNK was open for freeciv-2.5 development, and it has been part of freeciv project since. Freeciv-2.3 and freeciv-2.4 versions of the ruleset are still being maintained by cazfi on his own.
Alien ruleset is very different from standard rules - it has been one of the design goals of the ruleset that it should provide users with "second game" to play with Freeciv. We hope you can enjoy both more standard rulesets and Alien ruleset, not selecting just one of them to play.
All the information in the freeciv inline help when playing with Alien ruleset should be up to date (trust it over any other source). So should the information slowly collected to this wiki. Unfortunately these provide very limited set of documentation. There's more complete documentation from the time alien ruleset was still not part of freeciv at  but as it has not been updated since, it's partly outdated. There's some notes about the things that have been outdated in that documentation in the freeciv version control as doc/README.ruleset_alien.
Deneb 7 was known to have strange force field surrounding it that made it impossible to get near planet with year 250 Galactic Era Earth technology. However, when you were flying past, that field suddenly reverted and sucked you to the planet.
You find yourself in a strange world, probably touched by superior technology, where big portion of Earth science is invalid. You have to learn new rules, rules of this world.
There's deadly radiation that no known shielding works against. There's alien life, but more surprisingly also some edible plants just like on Earth. Radio doesn't work, air doesn't allow flying, some basic Physics does not apply here.
You struggle to live on this planet, and read Roadside Picnic by Strugatsky brothers once more.
City Size Limits
No city without Basic Infrastructure can grow bigger than size 2. For cities with Basic Infrastructure, planet's radiation limits how big they can grow. This limit can be increased by researching Radiation Resistance, Strong Resistance and Mental Powers technologies and by building Radiation Resistor and Skyscraper buildings or Protector wonder. Cities on Alien Forest or Radiating Rocks tiles have lower size limits than cities placed on other terrains.
For some reason it's hard to get water to flow long distances in this world. Irrigation is only possible on river tiles and tiles cardinally adjacent to oceanic terrain once Manufacturing technology is known. Water Flow technology allows us to irrigate tiles cardinally adjacent to city centers and river tiles. Once one knows Deep Pumping technology, one can irrigate any tile where terrain type allows it - no need for adjacent tiles to have water.
There is two veterancy levels for units: Regular and Elite. Regular units are promoted to Elite ones only rarely (10% chance) through real combat experience, and never by doing worker activities. One gets Elite units by building them in a city with Training Facility. Elite units have 100% bonus in combat compared to Regular ones. Team Gladiators units are built as Elite even without Training Facility.
Gold Upkeep Style
Gold upkeep for all buildings and units is paid in a lump sum after all cities have been processed. If the player does not have enough gold, random buildings from random cities are sold. If still more gold is needed, then random units with gold upkeep are disbanded.
Basic city radius square is 4 meaning that city can work all the tiles adjacent to center and one tile two tiles away in the middle of each side. This means that there is 13 workable tiles. Building Transportation increases radius so that 8 more tiles become available for total of 21.
Game starts at year 250 Galactic Era. Each turn equals one year.