I have four OD&D campaigns that share the same planet. The main campaign is called the Tarrozian Campaign and it is the most deadly of the four. The other three are more starter campaigns than anything else; they are where children and new players start.
I started the world from the bottom up and that is all I did for several years. Then I decided to do a little bit of top down just to define where we were. So what we have is a planet with an earthlike climate and gravity, but the surface area of Jupiter. (just keep thinking, "magic makes it work this way.” I played around with a few very rough sketches of the planetary surface and defined that the four campaigns are a long ways off from each other, a very long way. Even though we are now in the 45th (real world) year of the campaign and over 3800 game sessions in the campaign only a fraction of the world has been mapped or explored. We like it that way, knowing that there is really a vast unknown out there.
Here is a little bit about one of the things IMC where there was a diversion from the main campaign.
The players decided that they wanted me to start them out in a completely new area of the world, so they had no map, info, or anything and had to find out everything about where they were. At the time they had mostly 4th & 5th level characters with a 6th level magic-users IIRC. They told me this as we were starting a regular gaming session, so I told them I would think about it and have something completely new for them soon. And we started our game. They headed out and traveled to the dungeon that they were making their forays into and as they entered the first new room that they hadn't been in yet the door slammed shut behind them the room went pitch black and spun around holding them tight to the walls as it spun faster and faster and then the bottom dropped away from their feet (yeah, just like the carnival ride), then as it slowed down they started to fall and landed in the middle of a field in bright sunlight at sunrise. (I.e. they were a long ways to the west of where they were)
Well they spent a few days of game time getting acclimated to the area, which was complicated by a bit of a language barrier, which they had to overcome. They also were taken a ride on converting their money to local currency. They gradually learned about a massive ruin that was about a 2-day journey away to the southwest that no one ever returned from. They decided to go look anyway.
When they arrived, they found a ruin that was about a mile on a side, it was an enormous pile of rubble and as they searched, they found the remains of four city gates, one at the center of each side and a path that led into the ruins. Each path was similar and it was narrow with rubble piled high on each side. They picked one to try and as they crossed into the ruin onto the path, instead of seeing a path that looked like it had had some recent use, it changed to a path with deep piled dust that looked untouched for centuries. The rubble was covered with dust. This was quite odd since outside the ruins there had been recent rain, since they arrived, but the ruins, which looked weathered from the outside, looked untouched by the weather from the inside. They found through trying that they could not leave the path, well they could but it was like moving through cold molasses there was so much resistance to moving. They went in a little way and decided that they would go back outside camp and start in in the morning.
That is when they discovered that they could not leave because as they moved back toward the outside they begin to age. They first realized this when they started getting weaker and they noticed that each other’s hair was going grey at a rapid pace. They moved back toward the center and this reversed itself. They then saw that they had no choice at this point and set out toward the center.
(for anyone crying railroad, it is just giving them what they asked for, something completely new.)
As they continued towards the center the dust became progressively thinner until eventually it disappeared altogether. They tried turning back several times each time with the same results. They also continued to try to leave the path, especially after they barely survived several traps along the way, but could not do so. They also noticed when they looked at the sky that the sun flickered and seemed to move rapidly back and forth (not in the east-west direction but in the north-south direction. As they traveled further down the path it finally seemed that the sun was one solid sun several suns in width. They were unable to tell the passage of time as the sun (aside from its weird side-to-side movement) seemed to stay in the same place in the sky and not move. Nevertheless, as the traveled down the path they had to stop and eat many times, even though they did not grow sleepy or excessively tired. Even rationing the water, they were growing concerned about running out.
As they drew nearer to the center and rounded the last corner they could see a ruined tower in the center as they moved toward it, it started to become less ruined in appearance. In addition, the ruins around them started to look fresher and fresher as they went.
As they drew nearer to the center, they started to be able to smell smoke and hear the screams of women, the crying children, the shouts of men, and the shouts of the invaders. They began to see shadows of people all around them and things grew more visible as they continued.
They stopped several times studying everything they could see, and trying to plan for whatever was going to happen. They were a few feet from the where the path entered a large courtyard around the tower and they could see that it appeared to end at the courtyard.
They finally got to just outside the courtyard and stopping for a moment they made final arrangements and then stepped thorough into the courtyard and immediately stumbled over a huge pile of dead bodies, not of the defenders of the city or the invaders but men dressed as they were from their time, and before, hundreds of bodies (they realized) of all the adventurers that had proceeded them. All around the pile of the dead was another circle of the dead invaders that had fought these other-time arrivals. Several score of invaders guarded the base of the tower, a score of these invaders were struggling towards them over all the bodies, and they immediately retreated toward a wall and then back down the street that had replaced the path they had just traversed. The noise was deafening, with screams and shouts. Over it all, they heard a loud chant in an unknown tongue that the magic-users in the group recognized as a spell being cast. It seemed to come from the tower.
They hotfooted it around a corner and down an alley. It was chaos all around them. They made it several streets further from the center and then they saw a women furtively slipping along and just as she was entering a building a few invaders appeared (the invaders are at this point an unknown humanoid that seemed to be a strange mixture of reptile, insect and mammal)and quickly entered the same building.
They decided to follow them into the building and moving rapidly they entered the building and with all the noise where able to slip up behind the invaders and kill them. In the process of doing this, they freed the women whom the invaders had just captured. While a couple of the group secured and watched the doors, the two with the greatest charisma bowed deeply to the women and tried conversing with her. However, they were unable to find any language in common. Therefore, they resorted to sign language to try to ask her some questions, but after looking very doubtful and ill at ease, she then seemed to come to a decision and relax after no one tried to touch her.
She then signed for them to follow her and led them to a lower level and a secret door. They all followed her in and lighting a torch found several women and children. The women, a redhead, pointed to herself and they traded names, after this Taibiatonia (or Red for short) led them through passages sometimes down and sometimes up, a long twisting route. They often heard shouts and the clash of arms in the distance. Once they came face to face with a few invaders and rushed to close with them. They killed the invaders but not before sustaining several wounds among them. These were bound in a hurry and they continued along.
Red led them along and then she signed for quiet and for people to wait and slipped ahead, the stealthiest member of the group went with her and when they came back he told the rest of the group that they way out was blocked by a group of invaders and it looked like there were too many to fight our way thorough, but Red signed for everyone to follow her and after back tracking a little she led them out a different way. After passing through a choke point, the way started up at a steady angle. They walked like this for some time until the floor leveled out and led to a wall.
Red went to this wall and tripped a mechanism and it opened out of the side of a hill into a wooded glen. They closed it back and hid it again as best they could. She led them another mile to a hidden shelter and there they stopped. After eating and setting up watches they were looking back toward the city where it glowed way in the distance (and at a much lower elevation) from the fires and then all of a sudden there was a kind of pulse and then the city went completely dark. A few minutes later a terribly loud shattering sound was heard.
The senior magic-user stated that the spell that was being cast must have been completed and it probably explained all that had happened to the group since they had entered the ruins.
We played out about 50 game years (these characters and their children) in this new campaign area at the rate of about every other gaming session. They eventually learned the language and the lay of the land, about the invaders, married, had children and accepted that they were not going home. By the time, they reached the 11th-12th level they were able to inflict some major defeats on the invaders and drive them from the land by continuous guerrilla warfare, a lot of luck and by stealing some of the invaders magic. Eventually they were able to neutralize the spell on the ruins so that no one else would be trapped in the past and they were able to release hundreds of women and children that were trapped in their hidey-holes in stasis. The original characters were able to retire and then played on in this part of the world as some of the children and other characters. These later characters were quite bold and a number of TPKs occurred as they adventured far and wide.
They were eventually able to estimate (i.e. get enough hints out of me) that they had went back in time about 2500 years.