Thy purpose marriage, send me word tomorrow By one that I’ll procure to come to thee Where and what time thou wilt perform the rite, And all my fortunes at thy foot I’ll lay And follow thee my lord throughout the world.
Romeo will answer it.
Tybalt, the kinsman to old Capulet, Hath sent a letter to his father’s house
Act 2 Scene 4
A challenge, on my life
So smile the heavens upon this holy act That after-hours with sorrow chide us not.
Act 2 Scene 6
When Romeo climbs the dividing walls and goes to Juliet's balcony it affects the climax because if Romeo and Juliet had not been so quick to tell each other that they love each other and would marry each other the next morning after meeting that night. Romeo would not have been so conflicted when fighting Tybalt, to begin with since he would not have to think as Tybalt as part of his family and Mercutio would not be dead.
Since Tybalt sent a challenge to Romeo he had to respond and fight him because Tybalt makes every argument a swordfight. Tybalt ends up wanting to fight Romeo and this letter was his way of telling Romeo. Since later Romeo refuses to fight it leads to Mercutio's death and later Tybalt's death.
When Friar Lawrence finally agrees to wed Romeo and Juliet he foreshadows to the climax and what is going to happen later in the story. Since he hopes the heavens would be happy about the marriage and hopes nothing unfortunate will occur that would make everyone regret the matrimony.