Learn Finnish: Linnan muuri ja sen kaari olisi tuhottu. - The castle's wall and its arch would have been destroyed.

It uses the conditional form of the verb olla (to be), which is olisi, plus the past passive participle of the main verb (tuhottu). In passive structures, the helper verb is always exactly olisi regardless of who or what is involved.

The sentence mentions two things (a wall and an arch). Why does it use olisi instead of a plural form like olisivat?

In Finnish, the passive is completely impersonal. Even though 'the wall and its arch' translate as the subject in English, grammatically they are the object of the Finnish passive verb. Passive verbs never agree with the object's number, so the helper verb is always the singular olisi.

Why are the objects muuri (wall) and kaari (arch) in their basic nominative form, rather than taking an object marker like -n?

When a whole object is acted upon by a passive verb, it takes the nominative case instead of the normal accusative -n ending. Because olisi tuhottu is a passive verb structure, the items being destroyed stay in their basic dictionary forms.

What is the base verb for tuhottu, and how is this passive form made?

It comes from tuhota (to destroy), which is a Type 4 verb ending in -ta. To form the past passive participle for this verb type, you drop the final -a/-ä and add -ttu/-tty. This gives us tuhottu.