The duology featuring Deep Secret and The Merlin Conspiracy is set in the Multiverse (similar, in a way, to the Related Worlds in the Chrestomanci series). The exact number of worlds in unknown, but it has been mentioned a both novels that there are potentially an infinite number of these.
The Rules of the Multiverse
Ayewards & Naywards
The multiverse is defined in the opening chapter of Deep Secret as such:
"All over the multiverse, the sign for Infinity or Eternity is a figure eight laid on its side. This is no accident, since it exactly represents the twofold nature of the many worlds, spread as they are in the manner of a spiral nebula twisted like a Mobius strip to become endless. It is said that the number of these worlds is infinite and that more are added daily. But it is also said that the Emperor Koryfos the Great caused the multiplicity of worlds somehow by conquering from Ayewards to Naywards."
So the multiverse is in a figure eight, or in the shape of an infinite sign. Some of these worlds are magical, and some are not. One half of the worlds in the figure eight are negative magically. Those are the Naywards worlds. The other half is positive magically. Those are the Ayewards worlds. At the centre of the multiverse, situated across the twist is the Empire of Koryfos (or Koryfonic Empire).
Worlds are not fixed and can move either way. It is mentioned in Deep Secret that Earth is slowly being bumped Ayewards. It is possible for people from one world to spend time on another, though it is impossible to fully transition between two worlds.
Magic in the multiverse is kept in check by what is called "Magids" or the Company of Magids. Magids hail from both Naywards and Ayewards worlds and exhibit an aptitude for magic. Magids are powerful beings who essentially act as magical policemen throughout the multiverse and also handle different fatelines. Each world has its own magid or magids, though there is always a senior magic. Before he passed away, Stan Churning was Earth's senior magid. A Senior Magid is also mentioned as living several worlds Naywards from Earth.
The ability can run in the family, but it is extremely rare (this is the case for the three Venables brothers). Deep Secret goes much more in-depth on Magids though they are also present, in some capacity, in The Merlin Cospiracy.
If one Magid passes away, another is tasked (usually with the help of the deceased Magid, who can continue to exist in spectral form) with finding or 'sponsoring' a new one. Magids can perceive other people's magic ability, they are also all aware of each other and in contact in some capacity. Magids are kept in check, and given their missions, by the Upper Room. Who or what the Upper Room is exactly is never properly covered in the books but one can assume that they are the watchmen who watch the watchmen (members of the Upper Room are called Archons). The Upper Room is very knowledgeable on the past, present and even future of the multiverse. Though this ability is not exclusive to them, Magids also have the ability to go between the different worlds (though it can be difficult, Maxwell Hyde always has to get drunk before crossing through each world).
Exploring the Multiverse
Each world seems to be connected. How to access each different world seems to differ slightly between the two books, though it does seem to be slightly easier for Magids -assuming that this is because they are actually aware of each of the worlds.
Some of the worlds may share similar histories or caracteristics, for example Earth and the Isles of Blest do share some similarities (London exists in both). In a way they could almost be considered as parallel worlds. Other worlds are wildly different. The worlds are linked through what Nick describes as 'dark paths between the worlds'. It is unclear whether there is an official name for these paths, or if this is just what Nick views them as. There are also different ways of travelling between each world, for example in The Merlin Conspiracy animals (in this case, an elephant and a goat) seem to have a particular way of viewing and travelling between worlds.
It is unclear whether the dark paths are the only way to travel through each world. In Deep Secret it seems as if to travel through the dark paths, one must first help three people. However, this particular rule is not referenced in The Merlin Conspiracy and indeed both Magids and non-Magids travel through without necessarily helping others.
It is possible to accidentally go between the worlds without knowing why or where, and especially how. This happens to Nick Mallory a lot. Here are a few of the worlds (or parts of world) mentioned in the duology:
An important setting in both books. It is the home of various characters including Nick (and the rest of the Mallory family), Maxwell Hyde and Rupert Venables. This incarnation of Earth is very close to our own (both Bristol and London are mentioned, as are Tokyo and former Yugoslavia), and can be called Almost-England. It is considered to be a Naywards islands, or magically negative island. Nevertheless, it is the host to various magical beings and magic can happen there (as it does in Deep Secret). Though a Naywards world, it is the home of what is described as magical "nodes" (one of which is at Hotel Babylon, in Wantchester) which are points in which magic is very strong (and can be harnessed).
The Koryfonic Empire.
The Koryfonic Empire is made up of a series of conquered worlds, these worlds include Erath, Telth, Thalangia and Annergam. There are at least twelve of these worlds. The capital of the Empire seems to be in Iforion (it is where the palace is situated). There are fourteen languages spoken throughout the Empire. The Empire also has its own calendar and way of dating years, in this case with the initials E.K. At the start of Deep Secret, the year is E.K. 3413. It is mentioned that the Koryfonic Empire was already flourishing when Greece and Minos were new.
It is a futuristic Empire, there is mention of computers and teleportation. It is also an Ayewards world, perhaps not surprisingly given its location in the multiverse.
The Magid in charge of the Koryfonic Empire is Rupert Venables (who is also, at least partly, in charge of Earth). The Koryfonic Empire is well-known amongst Magids as a frustrating world to deal with as it is highly unstable and corrupt. The Empire is heavily featured in Deep Secret and much of the plot stems from the Empire's history and problems. It is only briefly mentioned in The Merlin Conspiracy.
The particularly of the empire is the presence of centaurs, whose allegiances are governed by very specific rules. Though rare, it is possible for centaurs and humans to mate (this is partly what causes the trouble in the first book).
The last Emperor of the Koryfonic Empire was Timos IX, though he was assassinated. He had various Imperial Wives (organised in various levels), including Lady Alexandra. There is a military, led by General Dakros and a mage, Junior Mage Jeffros. One of the greatest Emperors of the Koryfonic Empire was Koryfos the Great. There seem to be various religions in the Koryfonic Empire, one of which worships the bush goddess (also called the thornlady) Aglaia-Ualaia.
Is introduced in Deep Secret, a minor setting, it nevertheless plays host to refugees from the Koryfonic Empire during the course of the book. Furthermore, Magid Will Venables lives on Thule with his wife (Carina Venables) and their six daughters (Venetia, Vanessa, Vanda, Viola, Valentina and Vendela). Thule also has its own native flora and fauna, including a special kind of yellow Thule Quack (a type of duck or goose).
Whilst not technically one of the worlds in the multiverse, Babylon plays a big role in Deep Secret. Babylon in the novel seems to be Diana Wynne Jones' incarnation of Babel. To access Babylon, there is a special ceremony which every (or many) Magid knows a piece of (they each know two verses of the poem). Nick Mallory, Maree Mallory and Centaur Rob go to Babylon. It is described as a dark and trecherous place, and each person needs a set of specific items to get through. Once each person has gotten to the end of the path (or obstacle course), they are each granted one wish.
Babylon is also the Magid code name for Earth.
Isles of Blest.
Though different in some aspects (due to the difference in magic ability), Blest shares some similarities with *our* Earth, namely aspects of history (Stonehenge exists in both worlds, though is more developed in Blest) and geography (London exists in both, and the geography of the British Isles seems very close, if not identical). Rather than being united, each country (Scotland, Wales, England and Ireland) has a King.
The King of Blest travels around the country with the King's Progress (which include various courtiers, media and children such as Roddy and Grundo). The magical adviser to the King of England is the Merlin (not a person in this case, a title -similar to Chrestomanci). The Merlin is always male. The Lady of Governance is Merlin's female counterpart.
The Isles of Blest seem to be inspired by Arthurian legend as well as various Welsh legends. Mythical creatures such as dragons and salamanders exist there. Another particularity is that cities and towns are incarnated and guarded by anthropomorphised versions of them (e.g. Salisbury, London, Old Sarum).
They are one of the most magically strong worlds in the multiverse. In The Merlin Conspiracy it is explained that if the magic is affected in Blest, it will affect the magic in the whole multiverse. It is obviously an Ayewards world and the Magid in charge is Maxwell Hyde (who splits his time between Blest and Earth).
Mentioned in The Merlin Conspiracy, it seems to contain the English Empire and a Russian-Turkish block. Nick Mallory accidentally ends up here before meeting Roddy and going to Loggia City. It is seen as a bit of an oddity by those from Earth (especially from *our* England) as it is a world in which the English seemingly have an Empire expanding through Europe, in this instance France and Marseilles are mentioned. The monarch of the English Empire is not mentioned, though there does seem to be a Crown Prince who goes to Marseilles to play cricket. The Plantagenate Empire is an Ayewards world, as it is mentioned that the royal family is protected by mages. They are not described as being particularly strong, however.
Is described by Nick as "the weirdest city [he]'d ever seen". Loggia City is part of an un-named world and is introduced in The Merlin Conspiracy. The city seems to consist of a large arcade full of shops situated next to a huge ravine. The arcade goes up against the side of a cliff. Deep in the ravine there are trains and factories, and there's the other side of the city across the ravine connected by various iron bridges.
Loggia is situated in an Ayewards world, so witchcraft is very much a thing though seems heavily regulated by the Prayermasters (including the Prayermaster and the two prayer boys Joel and Japheth). Magic is tested by Dangerous Magic Talent assessment machines. Loggia requires an entry-permit and has a curfew.
Each floor of the arcade is devoted to something different. Lifts cost money. Level 11 A is residences (as are Level 12 A & B). Level 11 B is the House of Prayer for Holy Jazepta, College of High Prayermaster. Thirteen A is housing, and Thirteen B has a sign that says "SEX and DRUGS". Level 14 is less salubrious and has high levels of radiation. It also houses the Clerk of Public Works. Level 15 houses a Greasy Spoon which serves such food as scoppins, bindals, peslow, nipling, colly and klaptico. The very top floor contains as airfield and is where the weavers make their embroidery. One of the main caracteristics of the city is its reliance on the embroidery trade. Citizens of Loggia wear brightly coloured and embroidered clothes. The city also has a severe radiation problem.
We see Loggia City ten years in the future and as the weavers on the top level left the city the city's economy has ground to a halt.
Is briefly mentioned by Maxwell Hyde in The Merlin Conspiracy, though in no detail. It is unclear whether it is an ayewards or naywards world.
Isn't necessarily a 'proper' world in the multiverse, it is an islands composed of bits and pieces borrowed from many different worlds (mainly tropical it seems). The island belongs to bounty hunter Romanov and is heavily featured in The Merlin Conspiracy. It is said that there are parts of the island which are set in the future, a detail which is proven true by the end of the novel.