Introduction

Non-Player Character (NPC) and item placement has a direct correlation with the amount of immersion a map conveys. Let's assume you've designed a beautiful hospital and, for some indiscernable reason, you've placed a rocket launcher in the middle of a hallway. Even though it will still look like a hospital, it won't feel like a hospital anymore, and the player's level of immersion will be reduced significantly. If you replace that rocket launcher with a friendly doctor, it seems more real (because doctors are regularly found in hospitals) and thus the player's level of immersion will be increased.

NPC and item placement also has an effect on gameflow; if the player is given a choice between a hallway with a few strong enemies and another full of weak enemies, he has to decide which poses a bigger threat. Similarly, the player might prefer a room with some enemies and ammunition to one that has neither.

NPCs and items aren't the only factor in either of these situations, of course, but they're a large enough factor to merit some tips on how to properly place them.

NPCs (Allies and Enemies)

• Allies should be used sparingly. If you have access to them at any given time, they become taken for granted and used entirely too often for things they weren't intended to be used for, such as cannon fodder.
• NPCs should be placed in situations where they can be used toward their full potential; for example, a sniper is relatively useless if placed in a tight corridor. Alternatively, an alien with a powerful close-range attack will fight more effectively if it's placed in close quarters. However, most NPCs (with the exception of melee attackers) perform best in larger areas with plenty of cover.
• Some semblance of balance between numbers and power is necessary; that is, if one side has more powerful units, the other side should have more units (and remember that if you base your balance purely on allies and enemies, it would end up unbalanced as the player is his own strongest ally).

Items (Weapons, Ammunition, Health, and Armor)

• As was mentioned previously, weapons should be placed in environments suited for them; a rocket launcher shouldn't be found in a situation where a rocket launcher isn't useful or expected, such as an otherwise empty hallway.
• Try to deny the player access to powerful weaponry unless he has to struggle for them ( emphasis on the word "try;" this means it's not required). In singleplayer, this can be accomplished by placing the weaponry in an area populated with enemies. In multiplayer, you could place them in a hazardous (radioactive, for example) environment so he's forced to move quickly and carefully to get what he wants without getting too injured.
• Ammunition should be spread around in amount, type, and location; 5000 bullets in one spot isn't helping anyone if they can't carry more than 200 bullets and what they really need is shotgun shells. Exceptions can (and should) be made for armories/storage areas.
• Health and armor should be placed sparingly and rarely together, especially in multiplayer (because pillboxes just aren't fun).
• If mapping for multiplayer, try matching stronger weapons with incompatible ammunition so the player has to travel around to get what he wants (because pillboxes still aren't fun).

Note: If you're just starting with level design, it may be wise to start by making multiplayer maps so you don't have to worry about NPCs; later, once you're more comfortable with the entities, you can always make the shift towards singleplayer mapping.

Conclusion

Placing NPCs and items throughout a map to create cohesion in gameplay and atmosphere is one of the more subtle practices of level design, but one that can make all the difference in the world. A well placed weapon or health kit can change the entire pace and flow of a map, directing action and allowing for player strategies that will add depth and make your level more fun. Hopefully these tips can help you in creating more balanced and enjoyable creations.

Note: Please realize that all of these are guidelines, not rules... you can mix 'n match. The general idea is to make sure that your entities are there for a reason.