The use of the participe
passé is similar to the perfect participle in English. As an adjective it can be used in attributive form (as description to the noun) or predicative form (as part of the verbal description).
en 1856 sont en train de s' éffondrer.
The houses, built in 1856, were are about to fall apart.
predikative use = passive voice
Les maisons sont
construites selon les idées
The houses are built according to the ideas of Gaudi.
As mentioned before - here the participe passé matches in gender and number with the described noun.