World was a 'Nexus', a planet that had numerous gates and portals and thereby acted as a focus and attractant to wanderers from all over. The planet was colonized by a group of high tech species, but a disaster/war of some sort reduced the level of technology severely, and also cut the population down.
Around this time, local space stabilized and the portals shut down. The main worlds of the colonists became embroiled in a war and were overtaken by the race that was destroyed in the recent war (1000 years latter). So for nearly a 1000 years, the survivors and descendants of the colonists have been totally isolated, and the stories of magical technology and space travel are treated more as legend than fact.
Magic works. To keep it from being abused, you might want to consider the need for ritual, time to concentrate or collect energy for a spell may help. Magic is very much like any other form of energy. You can collect it, store it and use it up. We'll argue that all living things have some of this energy, and you can collect it store it etc. Remember to play the character, not the power as much as possible, though doing stuff for effect now and then is fine.
Technology works. On Nexus, technology is very low level (steam power in
some areas). The locals, which are a hodge podge of species from the colonial
time, are superstitious of technology. The few artifacts found killed off the
people who found them, and the belief that technology was the reason for the
disaster 1000 years ago makes it unpopular. So you can bring high tech to
Nexus, but you'll have to hide it. Overt use will bring a riot against you.
(need to fill in)
Things to control: I'd rather not go the MECHA route. And excessive use of guns / laser guns seems silly on Nexus.
This world is suddenly going to be thrust back into some of the real world. It is now along one of the safer space trade routes, and one of the few possible havens for a space port. Haven, which is the present trade center of the colony descendants, will have some people happy to start trade, others hoping to steal technology, and some that are not fond of the changes and the outsiders.
Big planet, lots of stuff can be found. Within the cities, you have a mix of races. The sentient ones will be primarily bipedal (I'd like to keep taurs as a 'What the heck is that sort of thing', if possible ;). Also, one or more avian races will be used as messengers between the cities.
Found on a semi-island Peninsula near the equator. Has a fair amount of rural lands around it for farming. Has major seaports in several areas, so it can trade with the few other cities on the planet. Other than ships, lizard drawn carts are the main form of transportation. Climate is basically a lush midwest with a damp, cool "winter", though the highland regions in the northern part of the peninsula, especially the mountains, get some snow. Setting is more central america like.
Haven has a large freshwater river called the Big Serpent on its southern
border, leading out into the ocean. There is a council of elders and
merchants who govern the city, with most of the more well to do families
represented, with the merchants/traders having the most power, along with the
scholar mages. Magic does work on Haven, but learning it from scratch is
difficult for descendants of a technological civilization. The Scholar mages
study a mixture of magic and some limited science, remnants of the old
civilization. Many of them shroud the limited technologies they have mastered
behind the veil of it being 'magic'. It is known that savages in the distant
regions of the world have a greater mastery of magic (though those savages
may have a culture millennia older than their own).
Most trade comes from the sea or river, as overland routes from other cities are pretty rough. (The rural areas DO travel overland to trade in the nearby city of course). The city is fortified with stone walls, but does not seem to be overly concerned with security issues, other than a small group of guards who protect the ships and merchant areas and patrol the city at night. From the city, the rural farm areas occupy a large part of the horizon, though plentiful wilderness can be found between the farms. Haven itself feels like a rural city, as the locals have kept the urban sprawl to a minimum by the use of 1-4 story buildings and the numerous small 'wilderness' patches that are their equivalent of parks.
The main port is on the Big Serpent below the city walls. The city itself is warm and friendly, though bustling with trade activity. A mixture of raw materials from Haven's rural farmlands and processed goods (many made in other cities) can be found in small open malls during the day. There is still a small market for the ancient artifacts, some of which do nothing, others flicker with lights, and others tend to kill their new owners.
There is a small underworld in Haven. Mostly thieves, though some part time assassins can be found. Murders are fairly rare, so getting a full time job as an assassin is a bad career move. There is the occasional preying on overland travelers. (If someone wants to create seagoing pirates, have fun. ;)
Varied, and mostly low key. As these were colonies from high tech worlds, the role of religion was never overpowering. Rumors of some secret cults always go around now and then, but no real evidence has ever been found.
Quadrupeds are considered to be animals, and stories of creatures that walk on all fours and talk are considered to be purely myths. (Of course, sapient quadrupeds and taurs DO exist on parts of Nexus, but they have avoided cities enough to keep the myth in place.)
Black skinned, pointy eared elf types are known in legend as the demons who used magic (technology?) to cause the disasters, and have since been blamed for nearly everything bad, though only pictures of them can be found, and no proof there were ever any on Nexus.
Legends tell of people falling out of the sky, or out of holes in the air. This has not happened for over a 1000 years, but enough of the planet is unexplored that when a strange creature is found, it is assumed to have fallen onto Nexus. With the recent war and damage to local space, the portals are now active again.
There is one northern city that uses friendly six legged felines called Ket'pilars for transportation. There are rumors that the 'owners' are very close to their animals, and one visitor swore they talked to their mounts and they seemed to answer. Preposterous. While drunk, he even claimed that they could use their front legs like arms, though even he said he only saw it happen once.
Again, varied. And you can create holidays and celebrations as you more or
less see fit.