Software never returned to the series, but did license the engine to numerous other titles before releasing the source code for free in 1995, and multiple other games in the Wolfenstein series have been developed by other companies since 2001. Gameplay Wolfenstein 3D is a first-person shooter presented with rudimentary 3D graphics. The game is broken up into levels, each of which is a flat plane divided into areas and rooms by a grid-based pattern of walls and doors, all of equal height. Each level is themed after Nazi bunkers and buildings. To finish a level, the player must traverse through the area to reach an elevator. Levels—ten in the original episodes—are grouped together into named episodes, with the final level focusing on a boss fight with a particularly difficult enemy. While traversing the levels, the player must fight Nazi guards and soldiers, dogs, and other enemies while managing supplies of ammunition and health. The player can find weapons and ammunition placed in the levels or can collect them from dead enemies; weapons include a knife, a pistol, a submachine gun, and a rapid-fire chain gun. While the levels are presented in a 3D perspective, the enemies and objects are instead 2D sprites presented from several set viewing angles, a technique sometimes referred to as 2.5D graphics. The player's health is represented by a percentage starting at 100, which is diminished when they are shot or attacked by enemies. If the player's health falls to zero, they lose one life and start the level over with a knife, a pistol, and eight bullets. The player begins each episode with four lives and can gain more by finding extra-life tokens or by earning enough points. Points are scored by killing enemies or collecting treasures scattered throughout the levels. Points can also be scored by killing all enemies in a level, collecting all treasure, finding all secret areas, or completing a level under a par time; the player's completion ratio and speed is displayed when a level is completed. Secret areas containing treasure, health refills, or ammunition can be found in hidden rooms revealed by activating certain wall tiles that slide back when triggered. The original version of the game allows the player to save their progress at any point, though in many of its ports the player can only save between levels. Plot Wolfenstein 3D is divided into two sets of