Minecraft wiki entry for Multiplayer_PvP_bases, associated text: There will be a huge shadow under your base, so someone right under it will know what's going on.
Liquid shields are very hard to make.
Scaffolding and ender pearls are commonly used to assault a sky base.
There are TNT cannons designed to shoot vertically to crush sky bases. To solve this problem, cover your floor with obsidian.