"The Wall" from Zbigniew Herbert - Help with interpretation.

My teacher assigned me this short poem for interpretation. This subject was never my strongest point.
I was trying to figure it out for a long time, but I just don't know what it might be about. When time came for me to talk about it in front of a class, after few minutes of awkward silence my teacher gave me another chance to prepare it for after christmas, but I still have no idea.

Poem:
We are standing under the wall. Our youth has been taken off like a shirt from the condemned men. We wait. Before the fat bullet will sit down on the nape of the neck, ten, twenty years pass. The wall is high and strong. Behind the wall is a tree and a star. The tree pries at the wall with its roots. The star nibbles the stone like a mouse. In a hundred, two hundred years there will already be a small window.
(I will have to do it in my native language though)

Will someone help me with interpretation, symbolism, or at least give me some hints on what it might be about?

Just for a start:

I should say, first of all, get a dictionary of symbolism in your own language.
Then read the poem several times until you get a general idea.
Then look for the meaning of the symbols specially the wall, the tree and the star.

I couldn't figure it out even with that.

All I can think of is that the wall might be represent some kind of barrier in life, that's it.
I'm not made for that.

If no one can provide an answer, maybe some more hints or something(would still prefer an answer though(lazy I know)).