A Six-Dimensional Multiverse

If we imagine that all of Poul Anderson's fictions coexist in a single multiverse, then that multiverse has at least six dimensions. Each universe has three spatial dimensions and at least one temporal dimension.

The universes coexist in a fifth dimension which is not temporal, therefore has to be a fourth spatial dimension. The Old Phoenix Inn exists between universes in this dimension.

The universes of The Corridors Of Time and of There Will Be Time definitely each have only a single temporal dimension. In each of these universes, a time traveller who departs from a particular moment in the single timeline either has already arrived at earlier moment or will arrive at a later moment in that same timeline. 

The universe of the Time Patrol series has a second temporal dimension. Some departing time travellers arrive at an earlier or later moment in the same timeline. Others arrive in a later timeline - "later" in the second temporal dimension. Thus, two Neldorian time criminals depart from a timeline in which Rome wins the Second Punic War and arrive in a later timeline in which they themselves help Carthage to win that war.

We imagine that travel between universes is relatively easy, relatively difficult or, alternatively, impossible, depending on the nature of the universes and of the precise relationships between them. We do not expect the two kinds of time travellers to meet. Universes where gods exist and magic works are usually insulated from those where only the laws of physics are applicable.