If I'm learning anything, these focused dungeons should probably be smaller, if not just a few rooms. I'll assume here that we have about one dungeon level, maybe 10-12 rooms. Remember, like the tumbling dungeon, the effective room number is multiplied when you apply the special effect. In other words a dungeon of 10 rooms and four time periods is really 40 rooms.
How to Travel Time
If the dungeon is designed to be visited through time then there should be some way to travel on site. You could have a mobile means of travel, say a ring. That would mean players could flick back and forth through the ages while standing right in front of a feature that they want to investigate through time. That seems interesting and convenient, it also seems like a pretty damn powerful item to let loose on your campaign. Every battle might be subject to a rewind. So I will settle on time travel through an unmovable location at the dungeon site. You might think, should we limit its affect to the dungeon itself to avoid that kind of havoc time travel might play on your campaign? I'm thinking no. If you put the dungeon in a remote enough place and the players want to travel all the way back there to try and use time travel to save a hireling or stop an assassination or something, well that sounds awesome to me.
The Time to Be Traveled
Something I managed to get right with my one previous go at time travel in my campaign was that I didn't let the players choose the granularity of their travels. In other words, they couldn't twist a dial and move back or forward 5 minutes. I used our game sessions as a unit of time. I think that idea of you the DM controlling where the party will be dumped in time is essential to this working, otherwise you basically have an infinite sandbox with no real way to prepare. It will also be too easy for players to just jump around obstacles instead of having to think.
So, I suggest limiting the time in our dungeon to be traveled to 4 eras. I'll name them after four stages of human age: infant, child, adult, aged. And I'm going to have to get to work so now some quick brainstorming on what things might be cool to have in each age:
- The architect of the place, can be persuaded to make some changes
- children to be saved that will apear later as traders or helpers
- places to plant seeds ala Ocarina of Time
- Completly different fauna, the beasts that had to be cleared out to build the dungeon.
- Some basements and parts of the structure started
- a real need for certain structures like bridges or stairs in a cliffside
- most rooms built, with furnishings, new and shiny
- people living here or using it
- perhaps the ruler of the place to interact with
- maybe different fauna besieging the place
- machines or contraptions that may break later
- fountains and magical devices that have very clear uses in the context of their time, baffling later
- the default time, first encountered by players
- just abandoned, broken furniture, ransacked
- stiff doors, bandits or unsavory types to interact with, hiding here
- different fauna, maybe vermin and parasites
- hard to reach spots behind portcullises or bricked up walls
- centuries after the place was being used
- ruined- ceilings collapsed, areas under water
- different fauna, maybe undead or those things that live in stark, barren places
- places difficult to reach without messing about with things back in time like shoring up ceilings or asking the architect to put in secret trap doors
- maybe a powerful hermit to interact with