You can unlock 13 maps (levels) from the classic Doom released in 1993. This page contains detailed information on requirements that you have to fulfill in order to unlock a classic map and where you can activate classic levels. The following pages contain tips on where you can find these classic maps - they are one of the types of secrets available in the game.
Every mission in the game (excluding levels with bosses) have one classic map. This means that you can unlock 13 levels in total.
Always start by searching for a lever - the picture above shows an example. Upon activating it you will also hear a characteristic melody that informs you about unlocking entrance to a classic map.
You now have to search for entrance. The picture above shows an example. It can be located near a lever or you may have to search for it a little bit further. Enter inside a classic map to make the game acknowledge the fact that a classic map has been unlocked. After that you can leave this "retro location" and continue the game.
Classic maps can be selected in the main menu - choose Classic Maps option. The list includes only those levels that have been unlocked in the single players campaign.