This grave is dedicated to all soldiers who fought in all wars where Poland took part, and there were many countries it was at war with. Why does it look like a ruin of a collumnade? Because it is a ruin of a collumnade! Before the WW2 there was a huge palace (look up "Saxon palace") on that square with the grave under the columns. Nazi were not keen on Poles expressing their patriotic feelings by going to the grave and placing wreaths and flowers on it, so they closed off the palace and then blew it up.
Instead of rebuilding the whole palace, it was decided to rebuild only the few columns under which the grave was. I think it looks very symbolical and elegant, just one unhealed wound on an otherwise totally recovered city.
If you own or manage Tomb of the Unknown Soldier (Grob Nieznanego Zolnierza), register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.