There is no real canon for Mausritter, but here's what I see as the "offical" view on some details of Mausritter's setting. This stuff can all be adapted, modified or subverted. If you've got more questions, let me know!
The setting of Mausritter is basically: it's the real world, but mice exist in a D&D-style fantasy medieval society and have access to magic. The players are the brave and desperate mouse adventurers who are willing to face down the threats and go to the dangerous places where the settled mice do not dare. It's asking the question "what do the D&D adventures in the Brambly Hedge world look like?"
The mice exist in a fantasy feudal world that takes inspiration from 14th century France, Italy and Germany, with plenty of 19th-century children's book nostalgia thrown in..
Mice mostly live in small settlements, often just a few families, but sometimes large cities of a thousand or more mice.
Politically the mouse kingdoms are pretty traditionally feudal. Kings, queens, earls, lords and knights. That sort of thing.
Some settlements are too small to even be protected by a local knight.
Many larger towns have bought or taken their freedom from the lord and are governed by councils of burghermice. Others are run by the tough tunneller's guild, or other guilds of crafts-mice.
The currency of the Mouse kingdoms are 'pips' — tiny pebbles, carved with an insignia of the Queen. One pip is worth 1XP to a mouse adventurer when taken from a place of danger and brought to a place of safety.
You can mention it, but details are forbidden.
Other animals exist on a spectrum of "just plain animal" to "complex society" to "mysterious, powerful and magical" depending on whim of the author.
Magic exists in the world, but is difficult for most mice to access and manipulate.
Mouse wizards are able to trap magical spirits or ghosts inside of obsidian tablets carved with runes.
Magic may take other forms, and can be a corrupting influence. Common mice know that wizard are to be feared and respected, but never trusted.