Successfully complete the game to unlock the cake title screen.
The graffiti in the game are clues for the ApertureScience.com website. Go to the site, and enter "LOGIN" as a command. Enter "CJOHNSON" as a username and "TIER3" as a password. After logging in, type "HELP" for a list of commands. In addition to logging in as "CJOHNSON" (Chell), you can login as anyone desired by typing a username longer than two characters and using "PORTAL" as a password.
While logged into the ApertureScience.com website, type "THECAKEISALIE" to access a hidden message left by some employees.
At the end of the advanced map for Chamber 18, you will have the same raised platforms as you did in the normal one except now after you leave the lower one, it falls into the toxic water. This forces you to be more precise with your portal jumps. However, you can get past this easily. Do the platform jumping as usual, but if you miss, just land on the platform you came out of. You should be able to fire an opposite color portal at the wall where you entered this area, allowing you to go back to the start. All the platforms rise again, and the portal is still where you left it. You can now fall into the pit again and be able to portal jump to the next ledge. This take longer but does not require precise timing.
On the level where you get the companion cube after you run past the energy balls and are in the room with the three elevators, there are two doors leading to a catcher and a platform that is pushed out of the wall. Walk up to the platform that is pushed out. Crouch behind it, and walk through the opening. On the left hand side are a group of pictures of people with boxes for heads and hearts around them.
If the "sv_cheats 1" code is enabled, achievements are disabled; however, there is a way around this. There are a few command strings that can be bind to keys which allow you to enable or disable God mode when desired, get all equipment such as weapons, plus the better portal gun. To do this, use the bind [key] "sv_cheats 1; wait; god; wait; wait; wait; sv_cheats 0" console command. For God mode, use the bind [key] "sv_cheats 1; wait; Impulse 101; wait; wait; wait; sv_cheats 0" console command.
Enter the options menu, select "Controls", then "Advanced". Turn on the option for the console. Then while playing the game, press ~ to display the console window. Type "sv_cheats 1" to enable cheat mode. Then, type one of the following codes and press [Enter] at the console window to activate the corresponding cheat function:
|Take damage, but cannot die||buddha|
|No clipping mode||noclip|
|All weapons, including those from Half Life 2||impulse 101|
|Enemies do not target you||notarget|
|Allow separate sets of portals to be created||change_portalgun_linkage_id [0-3]|
|Allow use of second portal placement||upgrade_portalgun|
|Portal placement cannot be bumped||sv_portal_placement_never_bump 1|
|Portals can be placed on almost any surface||sv_portal_placement_never_fail 1|
|All challenges completed||sv_bonus_map_complete 1|
|Spawn citizen||npc_create npc_citizen|
|Spawn Breen||npc_create npc_breen|
|Spawn zombie||npc_create npc_zombie|
|Spawn Bouncy metal ball||ent_create_portal_metal_sphere|
|Spawn Weighted Storage Cube||ent_create_portal_weight_box|
|Basic portal gun||give weapon_portalgun|
|Fire an energy ball||fire_energy_ball|
|Fire rocket from head (glitched)||fire_rocket_projectile|
|First person view||firstperson|
|Third person view||thirdperson|
|Set gravity; "600" is default||sv_gravity [number]|
|Disable cheat mode||sv_cheats 0|