The Tower of Mastery victory requires each mana sphere building to be built somewhere in your kingdom, for which you need access for each mana type except ice. After that, the Tower of Mastery itself can be built, again, in any city. All civilizations not at war with anyone else will declare war on you if you start the Tower of Mastery, so have an army ready when you start it. The mana sphere towers each offer distinct benefits on their own, as you can see from the articles linked to below. The Tower of Divination will give you a free tech, so you might want to start with it, so as to get a head start on Sorcery, which is required for the Tower of Mastery. However, depending on your starting mana, you will sometimes have no choice but to start with a different tower.
So it all comes down to securing the mana types and then devoting a bit of build time in a large city. Essentially, there are four ways of getting mana types.
- Grab mana nodes and unique features by territorial expansion. This should be fairly self explanatory.
- Rites of Oghma randomly spawns mana nodes everywhere in the world. You are not guaranteed to receive any in your territory but if it is large enough, you have a fair chance of getting almost enough of them.
- AI Players will not generally export mana types, but if you vassallize them, you can demand their palace's mana as a tribute.
- Reuse mana nodes - see below.
Removing sphere mana after a tower has been built has no effect on that tower, so you can return your nodes to raw ones with a Mage with Dispel Magic after the tower has been constructed. If you have the right palace mana you can manage to get all towers with few actual raw mana nodes that way. For Dispel Magic you need the Metamagic II promotion, for which you will need Sorcery and a raw mana node. By the time you get to Sorcery, your Adepts have likely been around for a while, so if you want to start building the towers as early as possible, you should build an Adept and get him up to level 3 with at least one pending promotion. That way, once you get the Metamagic node, you can give him Metamagic I, thereby elevating him to level 4, upgrade him to a Mage, and finally spend his free mage promotion on Metamagic II. If you guard this Mage well, you can even remove the Metamagic node now.
The Amurites get Metamagic mana from their palace, but they still need Sorcery as Dispel Magic can only be learnt by Mages. Still, this means that you do not need to worry about the Adepts' promotions, or about losing the only Mage that can cast Dispel. On the other hand, the Amurites' Cave of Ancestors gives your arcane units one XP for each mana type, so to max them out, it's really worth keeping most, if not all of the nodes.