I tested the idea of the tower of mastery being a huge mana magnet, removing all mana nodes from the world but giving 3 of each type to the player who builds it, playing with Amurites-OO
First, the 15 mana resources give all arcane units 15 xp as soon as they are built if the city has a cave of ancestors, which any wise player would build in their cities with mage guilds. This allows instant mages/conjurers from adepts.
Second, 3 of each mana will give all adepts all lvl 1 spells, and all lvl 2 spells to mages/conjurers, meaning that every adept you build is actually comparable to a level 30 mage in number of spells, giving incredible versatility.
Third, the mages/conjurers don't need many more xp to get level 6 and become archmages/summoners, who in turn can learn their lvl 3 spells quickly, and from any circles of magic.
Fourth, the other effects of mana add to the empires productivity. Spirit mana adds +15% gpp, mind mana adds +9% gold, science and culture and enchantment mana adds +3 happiness. This means that if you have size 20 cities that cannot grow anymore because of lack of happiness, their productivity can grow 15%, or 15 gold/science with cottages or 12 hammers with mines with blasting powder. Add here the library, money changer, tax office, basaar of mammon, crown of akharien, forge bonus to get 34 beakers, 37 gold or 13 production, with 10 such cities it makes 224 science, 257 gold or 130 hammers, meaning that averagely you get 2 super adepts every turn extra. With mind mana it's 244 science or 281 gold extra.
Fifth, the mage heroes get a huge bonus. Hemah gains all lvl 2 spells and 5 promotions extra and the hero promotion allows very quick leveling to twincast. Govannon gains the same spells and the promotions can increase raw combat power to gain extra xp from enemies weakened with maelstrom.
So who needs vassals or allies anymore?