Learn Hardcore Turkish: Güvenlik protokolünü tanımlayan mekanizmalar kuruldu. - Mechanisms defining the security protocol have been established.

Because it’s the definite direct object of the verb tanımlayan in the relative clause. In Turkish, definite objects take the accusative suffix (here -ü).

What is tanımlayan and how does it function in this sentence?

Tanımlayan is the active participle of tanımlamak (“to define”). The suffix -an turns the verb into a modifier, so güvenlik protokolünü tanımlayan means “that define the security protocol.”

Why is the main verb kuruldu in third-person singular when mekanizmalar (“mechanisms”) is plural?

Because kuruldu is in the passive voice. In Turkish passive constructions, the verb normally stays in third-person singular regardless of the subject’s number. Here kuruldu means “were established” or “were set up.”

How would you rewrite the sentence in active voice and specify who set up the mechanisms?

You could make the agent explicit and use an active verb form. For example:
Mühendisler güvenlik protokolünü tanımlayan mekanizmaları kurdular.
Here mühendisler is the subject and kurdular agrees in number (third-person plural).

In English we’d use a relative pronoun like “that” or “which.” Why isn’t there one in Turkish?

Turkish doesn’t use separate relative pronouns. Instead it attaches participle suffixes like -an (active) or -dığı (passive) directly to the verb. Thus tanımlayan alone conveys “that define.”

What if only one mechanism was set up? How would the Turkish sentence change?

You’d switch the noun mekanizmalar to singular:
Güvenlik protokolünü tanımlayan mekanizma kuruldu.
The passive verb kuruldu remains the same (it does not agree in number).

Why doesn’t güvenlik take any suffix before protokolü (i.e. güvenlikin protokolü)?

When one noun modifies another as an attribute in Turkish, the first noun stays bare. So güvenlik simply qualifies protokol, forming the compound “security protocol” without a genitive or possessive suffix.

Is the word order strict in this sentence? Could parts be moved around?

Word order in Turkish is relatively flexible, but modifiers must stay directly before the words they modify, and the main verb typically comes at the end. You could front mekanizmalar for emphasis, but you cannot separate tanımlayan from mekanizmalar, and kuruldu needs to remain last.