Mapping For Hidden - A Beginners Guide
Some guidance by boxy off the hidden source forums.
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Some of you are struggling to get mapping for Hidden. Well, as said before, grab the latest install and you will find the FGD in the bin folder. This can be put wherever you like, as you point Hammer at it via the options-> configuration menu. Once mounted in the same way as other mods and the HL2 stuff, you will have the following extra entities to play with:

info_hidden_spawn (Point Entity) - The Hidden spawn point. The Dev team recommend around four spawn points for 617, less and OVR maps get crowded, but you can put more in if you wish, depends on map size. No important flags or fields.

info_marine_spawn (Point Entity) - The IRIS spawn point. We recommend you place down eight, no less and CERTAINLY no more. This is a nine player game at its maximum! Again positioning is upto the level designer. No important flags or fields.

info_spectator (Point Entity) - This is the camera point, and it will also be the position of the actual model camera in the world. The team recommend you only put enough in cover as much of your map as possible, as cycling through camera after camera to find the action can get tedious. Most of the official maps have at most eight or nine cameras, less if possible. No flags but be sure to give the camera a name for the area its looking at and adjust the FOV if you want the camera to cover more or less space.

hdn_ammocrate (Point Entity) - This is the ammocrate used in the OverRun game mode. DO NOT use this in standard game mode "HDN_" maps! Place down ammo crates at 'suitable places' in your map (i.e. not at end of a dead end so IRIS can camp it!). The team recommend two maximum per map.

marine_clip (Brush Entity) - This is basically the same premise as the player clip, except it only blocks the IRIS team from going through it, Hidden will pass through as if nothing was there. This is great for sealing off areas such as vents that you dont wish the IRIS team to go into and camp, etc. Try to use a model or some other reason for why the IRIS cant go to a location, rather than just throwing a brush in to block them, although this isnt always possible. USe the nodraw texture on this brush.

location_brush (Brush Entity) - This is where the HUD gets its location information from. Fill out the name of the location in the field and when the player passes through this brush the location is updated on the HUD until they touch another location brush. Two schools of thought, the room size brush or the portal brush. We went with the 'portal' approach on our maps, meaning you have two brushes at each door, so that as the player passes through either way the hud gets updated with location info. Use nodraw or trigger texture on this brush.

hex_extraction_point (Brush Entity) - This designates the location the IRIS team must get to and hold for a predetermined amount of time to signal round end. It should be tied to the objective points in the map so that it only appears/becomes active once all other objectives are complete. No special flags but it is a flexible brush, allowing the designer to set a name by which other entities call this one, the amount of time the area must be held by IRIS for extraction, the percentage of the alive team that must be inside the area to trigger the countdown till extraction, a failure message to reinforce what players must do to get the extraction point to 'work', and finally a toggle for whether the brush is visible or not. There is a gen_evacpoint_brush texture for this brush but you may use any texture you wish.

hex_timer_button (Point Entity) - This essentially works the same as a func_button but with a timed delay and must have a model tied to it. An example usage of this button would be on locks or terminals; the time taken to 'use' the button could be the time it takes to bypass security features or hack a terminal for instance. No special flags but is once again quite flexible. It allows the mapper to assign the amount of time it takes for the button to be used, a reset delay so that if the button is started but not used for long enough it can be reset if required and finally a filter for which team can use the button (Hidden only/IRIS only or Both).

hex_damage_button (Point Entity) - This works the same as a func_button but with a damage delay and must have a model tied to it. An example usage of this button would as a main server or terminal that IRIS must destroy before evacuating. The entity triggers outputs upon hitting 0 health. No special flags but is once again quite flexible. It allows the mapper to assign the amount of damage it takes for the button to be triggered and a filter for which team can damage the button (Hidden only/IRIS only or Both).

hex_logic (Point Entity) - This entity defines the logic of the gametype. It allows the mapper to control how many respawns the Hidden player has within each round, whether the Hidden player starts with all his weapons and whether the IRIS team start with all their weapons. This allows for life control, dependent on map size as well as allowing for either/or both teams to have to find weapons (such as an armoury) before combat can commence.

hex_pickup (Point Entity) - INCOMPLETE To be detailed at a later date

Mapping Tips

To test a level out, boot it up in Hidden and bring down the console, default button ` (tilde). Type sv_cheats 1 and then bot_add.

DONT USE FOG, it makes the Hidden stand out like no ones business! Sos, thats a bit of easy atmosphere you cant use!

Sprites are occluded by Subject 617 when he walks in front of them, bare that in mind when banging sprites all over your map.

Physics objects are what makes the Hidden that much more fun than some other mods out there, but don't go mad, you will lag servers and crash them if you go over the top, Source has a cap on how many entities can be in a map.

Make sure you have at least ONE hidden, ONE IRIS spawn and ONE info_spectator before you test a map, or it will just crash when you load it up.
*added notes*
-We used the trigger texture for our location brushes, its transparent and still works in game with no compile problems... -boxy
-we only provide for the locked in place spectator cameras, so that's all you can use. -ging
-No, fog doesn't play nicely with the shader that the hidden uses - if you put fog in, he becomes basically visible as the fog isn't drawn behind him.-ging
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TUTORIAL END