The speaker sees no alternative to death, like surrender or escape. This is a very desperate situation.
Making their mock at our accursed lot. (4)
The speaker sees this death as heroic, because death has been handed to him by chance or destiny.
If we must die, O let us nobly die, (5)
These phrases put two opposite feelings together: powerlessness and powerfulness.
Shall be constrained to honor us though dead! (8)
This could be lifted straight out of the Iliad or another Greek heroic epic, because the myth was that your afterlife lasted as long as living people remembered you.
And for their thousand blows deal one deathblow! (11)
This is a very macho attitude towards death. The speaker does not intend to go down a victim.
What though before us lies the open grave? (12)
This is the scariest line in the poem; Shakespeare would have used something similar in the climax of one of his tragedies. The sentiment here is that, since they're absolutely going to die, they might as well go down fighting nobly.
Pressed to the wall, dying, but fighting back! (14)
Again, the speaker knows how the fight will end, but he doesn't intend to give himself up to feelings of defeat.