Learn Dutch: Het was de held die de stad stichtte. - It was the hero who founded the city.

This is a cleft sentence, which is used to put special focus on a specific piece of information. By using the structure Het was ... die ..., you strongly emphasize that the hero did it, rather than someone else.

Since de held is a person, why don't we use wie for 'who'?

In Dutch, you do not use wie as a standard relative pronoun just because you are referring to a person. Instead, you must look at the noun's article. Because held is a de word, you use die. You would only use dat if it were a het word.

Why is the verb stichtte at the very end of the sentence?

The second half of a cleft sentence (die de stad stichtte) is a relative clause, which follows the exact same word order rules as a subordinate clause. In Dutch, that means the conjugated verb gets pushed to the very end.

Why does stichtte have a double 't'?

The infinitive of the verb is stichten (to found). To make the simple past tense, you take the stem sticht and add the past tense ending. Because the stem ends in 't' (which is in the 't kofschip memory rule), you add -te for the singular, resulting in stichtte.