This small building can be best described as the assembly hall for the lower classes of your cities; it is where the decisions of government as well as resolutions to engage in conflict are passed.
For this reason, the Peasants' Commune serves as the rallying point for melee levy units. While they aren't as powerful as the more professional units raised from the Barracks, they are still useful, first as a meat shield, and secondly, they can be used to man towers, castles, and cities to boost their fighting strength.
The Portuguese Peasants' Commune has a design that is distinct compared to other factions, and has better stats: it has higher armour, a limited ability to garrison units, and is built faster.
For most of the world's population, life continued to be harsh and challenging throughout the Middle Ages. Necessity bred the need for cooperation and interdependence, forming bonds between individuals, especially amongst the working classes of mediaeval society everywhere.
Mediaeval communes were compacts of mutual collective defence between the members of the geographic location covered by them. Members of the commune swore oaths in public to defend their fellow-citizens as well as their communities — an attack on one member was seen to be an attack on all. Although communes in Europe were associated with town-dwellers, arrangements such as these were also to be found amongst the rural peasantry.