At least means like "the lowest in importance or rank" and unless is like "except for ..."

At least has a few uses, all very different from the word unless here are some examples http://dictionary.cambridge.org/dictionary/british/at-least unless = if not Unless is similar in meaning to if not and can be used instead of if not in certain types of conditional sentences. We normally use unless with present tenses when we are referring to the future: You won't get in to see the show, if you don't have reserved seats. OR: Unless you have reserved seats, you won't get in to see the show.