Learn Russian: Сегодня гид ведёт свою группу в эту крепость. - Today the guide is leading his group to this fortress.

The verb вести (where ведёт comes from) is a unidirectional verb of motion. We use it because the guide is making a single, specific trip in one direction to the fortress right now. If the sentence was about a repeated habit, like leading tours every day, we would use the multidirectional verb водить.

Why does the sentence use свою instead of его for his?

The word свой (which becomes свою here) is a reflexive possessive pronoun meaning one's own. It always refers back to the subject of the sentence. Since the guide is leading his own group, we must use свою. If we used его, it would mean he is leading some other person's group!

Why are the words свою группу in the accusative case?

The verb вести (to lead) is a transitive verb, which means it takes a direct object. The group is the thing being led, so the nominative своя группа changes to the accusative case свою группу.

If в эту крепость is in the accusative case to show direction, why doesn't the word крепость change its ending?

The word крепость is a feminine noun that ends in a soft sign. For this specific group of nouns, the accusative case looks exactly the same as the dictionary form. We can tell it is definitely accusative because the pointing word changes from эта to эту to match it.