Levels can be connected via Links that will instantly transport the player to another position on the server once touched. Most servers feature an overworld where almost every level is linked to four adjacent ones. Previously, levels were also optically linked to appear smoother using Bigmaps, which is now deprecated in favour of GMaps which make the whole overworld appear as one single levels in even more ways.
Previously, level designers often used Baddies as standard foes for the player wherever such was needed. Therefore these Zelda-A-Link-To-The-Past-like soldiers and monsters were often seen all over the place. Now, this practice is deprecated and developers should create customly scripted NPC baddies which work better when multiple players are engaged in battle at once, and also they usually provide more fun to players.
NPCs, finally, are objects that hold scripts that define their behaviour. Using them, a developers can emulate all other components of a level, and of course more complex things such as characters a player can talk with, elaborate traps and enemies, graphical effects, and the like.