Is it allowed for you to not defend with any creatures at all when the opponent is attacking (even if you have untapped creatures) and take the damage yourself?
For example, I have a sliver deck and I want to build up on slivers to take down my opponent, but my opponent attacks me and the creature she uses is capable of taking out my sliver. Am I allowed to ignore that creature and not assign defenders to defend the damage and just take the damage myself?
You are not forced to block attacking creatures.
There are certain creature abilities that force you to block, but generally, you can leave attacking creatures unblocked, and they will deal their damage to you instead of your creatures. I suggest you read the Basic rulebook:
Your opponent chooses which of his or her creatures will block. Tapped creatures can’t be declared as blockers. For blocking, it doesn’t matter how long the creature has been on the battlefi eld. Each creature can block only one attacker, but multiple blockers can gang up on a single attacking creature. If that happens, the attacking player orders the blockers to show which is fi rst in line for damage, which is second, and so on. Creatures don’t have to block.
