You want to make a single player map but overwhelmed by the thought of adding a plot, placing weapons and monsters and generally doing lots of hard work? Dont be.

Weapons And Supplies
Players should have to generally work for supplies and the size of the task should match the supplies they receive because if a player takes down 5 monsters and breaks into a room to find just 1 health-pack they won’t be happy. Essentially the amount of effort involved should determine the supply the recieve.

Gone are the days of having to find supplies in crates as well. Use them sparingly. Not only is it frustrating to have to break through 5 boxes to only find one ammo clip its unrealistic and unimaginative. Use shelves, tables, even floors to provide supplies (as well as picking them up from baddies).

Ammunition and health is tricky to get right and really requires a bit of playtesting to get the balance. The average player who has not been wasting ammo or doing suicide runs everywhere should generally be getting a steady supply of supplies throughout the level.

Stronger weapons should be given out later on in your levels. They should also be placed in believable places that fit the use of the weapon. For instance, the sniper rifle should be picked up in/around/near a large, outdoor area and not some claustrophobic box room (note how this is the opposite to multiplayer maps). It is also important that there should be a reason to give the player a new weapon. Taking the sniper rifle as an example again, it would be logical for the player to expect to deal with other snipers or be involved in some other long distance combat within a reasonable time after getting the weapon. Weapons can also be given out to deal with an increasingly difficult enemy presence. In Half-Life you got the machine gun when you began facing the new threat of grunts, you get the rocket launcher when you have to face flying craft.

Map Structure
Single player maps need a fairly linear route to guide the player through the levels. Having a few different routes leading to different locations around your level may seem like a good idea but its more than likely players will waste their time backtracking and exploring those routes they haven’t been down. Remember that you are building a FPS level not something for a RPG game and the player needs to feel he is continually advancing forward in terms of plot (see gameflow below) and action.

Backtracking, if used in moderation is a good way of keeping a linear level without having to build extra parts of the map. An obvious use would be to turn off the security system to access the main door. The main advantage of this backtracking/looping is that the player is given their goal straight off (in the example, it would be activating the main door) so they know exactly what they have to achieve. Do bear in mind however that the player essentially has to go through the same area twice which can get boring quickly if overdone.

Throughout your maps give the impression that there is more to explore outside the playing area even though you might not technically be able to get to it. Your level should not look/feel like it is in its own void. As an example, If you are in a city, you could have high rise buildings in the distance (Half-Life 2 is a prime example, it even has the big citadel dominating the skyline). If you have an office level, you can have numerous rooms that are locked but you can see in through the windows. By placing obstacles in certain places, obvious routes around your map can become not so obvious ones, breaking up the strict, linear route but still guiding the player around.

Every realistic location has some history attached to it and this should be reflected in your maps. It should not look like it was just built for the player to run around. If, for instance, the player is going through an abandoned building there should be signs that at least give the player some hints as to why it was abandoned. Was it too old, was it unsafe? Have holes in the floor, collapsed walls and old decor to reflect this.

Your levels should have some sort of continuity. The theme of your maps (whether it is on a spaceship, terrorist base or whatever) should run throughout. Build your maps accordingly and choose suitable textures. If you do go from one area to another the change between the areas should be smooth and believable. For example, players shouldn’t just be in a sewer one moment, and then open a door and find themselves in a casino. They should find themselves in the lower levels of the sewers, then the higher areas, then maybe the maintenance sections, the basements and finally the casino proper.

Gameflow
Your single player maps should have an overall goal which gradually gets revealed to the player and throughout it there should be mini missions or diversions that relate to the overall plot. Think of films as an example. In James Bond films he has to stop the evil bad guy from taking over the world but first (speaking in very general terms) he does some shadowing, blows up a few buildings, kills a few henchmen etc. In Half-Life 2, you start off knowing very little of whats happening or where you are, but soon enough you are introduced to your first mini mission of escaping City 17, getting to Black Mesa East and meeting Eli, who can further expand the plot for you. A level without any expanding plot will struggle to pull the player in. They need to feel there is a purpose to what they are doing.

To go along with having a plot to your levels, have a decent ending to accompany it too!

Vary the action in terms of pace and challenge. Few players will enjoy the prospect of constantly blasting away enemies or doing puzzle after puzzle. It quickly gets repetitive and frustrating for the player. For every section of action in your map, the player should be given some form of break afterwards. If the player has just been through some major battle a period of respite should follow.

Baddies
Never just have monsters hanging around doing nothing. They should be active/responding to events. It can be as simply as soldiers patrolling or guarding. There should also be a reason for baddies to be doing what they are doing and being where they are. If you come across a soldier just stood around on a subway, for all the player knows he could be waiting for his Auntie to come visit.

The player should feel like he can be attacked from any angle at any time. If enemies run in from the front all the time it becomes repetitive. However have some sign that enemies will come from other angles (an exploding door is a good example) as it is unfair to be hit when you don’t know anyone is there.

Other Points
Place autosave points in your maps just before points where there is a high risk of dying, like where there are a number of dangerous jumps to be made or there is a boss monster. There is nothing more annoying than having to restart from a long way back. Even if you want to challenge the player, this is not one of the ways to do it. Don’t rely on the player to save the game for himself.

Never let the player feel they have died unfairly. The worst case scenario is trapping the player in a situation where they have to restart/kill themselves to escape. Make things hard but fair, dont have the player face off against dozens of enemies with only a pistol.

Single player levels with all the scripting and different interactions are heavily reliant on timing and tuning. More so than multiplayer maps you have to test, test, test until you have the right levels of ammo for the weapons the player has, the right weapons to deal with the enemies that are thrown at him, the right position for him to set off a scripted sequence, etc etc.