When constructing encounters, one of the easiest ways to add interest (and one of the easiest things to gloss over) is the terrain. There's only so far monsters and NPCs alone can get you, especially if your players have been around long enough to know the Monster Manual backwards and forwards. Beyond the standard (and therefore rather boring) difficult terrain, there are a number of simple terrain features you can use to spice up nearly any encounter.

Adding levels to any terrain is easy and forces the players to think in more than two dimensions, and, if done well, can lead to the party becoming split, which is often dangerous and, thus, fun. There are many ways to manage this this: Put snipers in trees in a forest, have ninjas jump in from the second story of an adjacent building, have trolls attack from beneath a bridge, whatever your imagination conjures forth.

There are several ways to go beyond the simple addition of levels, however. After all, levels are little more than a simple barrier if the PC aren't forced to interact with them. Pit traps are an easy way to force this interaction. In one dungeon I designed many years ago, there was a tomb with an aging stone floor that had a chance of collapsing under their weight, dropping the PC in question into the carrion crawler-infested area beneath.

More than just traps can force the PCs' hands though. Simple placing the PCs' goal on a different level than they begin on is more than enough. In one adventure Michael DMed, the PCs had to scale a mile-tall tower to its apex, the last stretch of which was a narrow walkway that spiraled around the outside of this tower. And we had to do it while a flying ooze attacked us.

One 'level' that is nearly always available is the space above the ground. Of course, only creatures capable of flight are capable of reaching this space normally, and as most PCs won't have this capability, be careful how you use this space. Try to add additional terrain features (trees, easily-climbed cliff faces, etc.) that allow the PCs to join flying foes in this area. The flyers will still have the upper hand, but it'll avoid leaving the players feeling useless.

I'm sure I'm not the only one with harrowing tales of multi-tiered battle, so let's hear your war stories in the comments.