I have a sky house made out of hellstone, but the hard mode monsters like wyverns get in and kill me. How can I defend against them?
There's no way to defend your house from the Wyvern, it can travel through blocks and no traps, even lava or spear traps, can kill it fast enough to prevent it entering the house.
However you can stop it from spawning which effectively solves the issue.
You can do any of the following to prevent the Wyvern from spawning:
- Build the house at a lower altitude. See here for minimum spawning height
- Have at least one town NPC near the player
- Make an artificial Crimson, Corruption or Dungeon Biome