Learn Danish: Bygningen er meget seværdig fordi arkitekturen er unik. - The building is very worth seeing because the architecture is unique.

It is created by taking the verb stem se (to see) and adding the suffix -værdig (worthy of). This is a common way to form adjectives that evaluate something, literally meaning "worthy of being seen."

Would seværdig change if the sentence was about multiple buildings?

Yes. Because it acts as a standard adjective describing the subject, it must agree in number. If the subject were plural (bygningerne), you would add an -e to the end, making it seværdige.

Why doesn't the word order invert after fordi?

Fordi introduces a subordinate clause, which uses the rule of Subject - Adverb - Verb. Because there is no central adverb (like ikke or ofte) in the phrase arkitekturen er unik, the subject (arkitekturen) simply comes directly before the verb (er), making it look identical to a main clause.

Could I use the word enestående instead of unik here?

Yes. Unik is a direct loanword and perfectly natural, but enestående (literally "one-standing") is a very common native Danish synonym often used to describe remarkable architecture or experiences.