Centaur Aisle

It's not easy being a substitute king in Xanth. a land where evryone has the ability to work a magic spell. But when King Trent and Queen Iris leave for a much-needed vacation, the young magician Dor has to take charge, reluctantly, governing the centaurs, golems, ogres, medusas, and other inhabitants of Xanth.
When Trent and Iris fail to return, Dor and his firends Smash the ogre, Grundy the golem, Chet the centaur, and the beautiful (but difficult) Princess Irene fear that something terrible must have happened, and they set out on a desperate rescue mission. During their peril-fraught search through Xanth, King Trent appears in a dream and directs Dor and his party to medieval Mundania, where the royal couple are being held prisoner. And only a mysterious power called the "centaur aisle" can release them....
Mundania, however, a land of ordinary things, lacks magic, and the powers of DOr and his friends will be useless there. Moreover, even if they can rescue the royal family, ther ewill still be the problem of returning to Xanth.
And what in heaven's name is the centaur aisle?