Sky City

Sky Cities are the best defense in Hardmode against the Corruption and the Hallow for your homes and NPCs. They are completely detached from the surface and when lit properly, protect against almost all enemy spawns. It is best to use non-corruptible blocks for its construction.

Safe heights
Levels are determined based on an average surface level, to prevent Harpy and Wyvern spawns. The surface may be higher or lower in your building location. For taller cities (100+ blocks) lower levels would be required. Low Sky Cities can be built by mining away more surface blocks, essentially making the ground lower.