Why were only monosyllabic words permitted within the walls of montasery?

There is no record of a place called "Montasery" nor any rules prohibiting the use of words longer than one syllable within its walls. This sounds like a fictional place or a made-up rule.

It's possible you are thinking of a specific story, book, or film where this rule exists. If you can provide more context, I might be able to help you understand the origin of this rule.