Learn Croatian: Svaki turist želi vidjeti najstariju kulu. - Every tourist wants to see the oldest tower.

It uses the superlative prefix naj-. To make a superlative, you first take the comparative form of star (old), which is stariji (older). Then you add naj- to the front to get najstariji (the oldest). Finally, the ending changes to -u to match the grammatical case of the noun it describes.

Why do najstariju and kulu both end in -u?

They are the direct object of the action -- they are what the tourist wants to see. This means they must be in the accusative case. The dictionary form of the noun is kula (a feminine noun), and feminine nouns ending in -a change to -u in the accusative. The adjective must match, so najstarija becomes najstariju.

Why does the sentence use the infinitive vidjeti (to see) right after želi (wants)?

When a person wants to do something themselves, you simply put the second verb in its dictionary form (the infinitive). Because the tourist is the one wanting AND the one seeing, želi vidjeti is the standard way to express this.

Does the word svaki (every) change depending on what comes after it?