Learn Italian: Furono signori crudeli per i cittadini del territorio. - They were cruel lords for the citizens of the territory.

Furono is the third-person plural of the passato remoto (historic past) of essere. This tense is used for events or states that happened in the distant past and are completely finished, with no connection to the present. Because this sentence is talking about historical rulers, the historic past is the most natural choice here.

How do you pronounce furono? Which syllable gets the stress?

The stress falls on the first syllable: FU-ro-no. It is very common for learners to accidentally stress the middle syllable, but for the passato remoto forms ending in -rono, the stress usually falls on the third-to-last syllable.

Does signori always mean 'lords'?

Not always! In everyday modern Italian, signori usually means 'gentlemen', 'sirs', or 'ladies and gentlemen' (when mixed). However, in historical contexts like this sentence, un signore refers to a feudal 'lord' or 'ruler'.

Why does crudeli end in -i? Is it just copying the ending of signori?

It happens to match here, but that is because the singular form of the adjective is crudele. Remember that adjectives ending in -e in the singular change to -i in the plural, regardless of whether the noun they describe is masculine or feminine.