Liero, Finnish for earthworm, was first released by Finnish programmer Joosa Riekkinen in 1998 using MS-DOS, and has been described as a real-time version of Worms. It has many weapons and sounds from its precursor, MoleZ, and provided inspiration for the game later game, Soldat.
In Liero, two worms fight each other to death for score (or frags) using a choice of five weapons from a total of 40 in a two-dimensional map. Most of the terrain, except for indestructible rocks, may be dug or destroyed by explosions. In addition to the weaponry, each player has a ninja rope which can be used to move faster through the map. This grappling hook-like device substitutes for jetpacks and can even latch onto the enemy worm to drag him closer to his foe.
While playing, there are health power-ups to heal your worm. It is also possible to replace one of your five weapons by picking up bonuses. Before playing, you can select certain weapons to be available only in bonuses, in the entire game, or completely disable them.
Unlike most side-scrolling deathmatch games, the weapons in Liero have infinite ammo. Key factors include the reload rate of each weapon and how fast they shoot, whereas in most others include how much ammo a gun sports and how frequent more ammo for that gun can be found. Liero depends all on timing and swift maneuverability.
The gameplay mode can be deathmatch, Game of Tag or Capture the Flag. It can be played by two human players simultaneously in split screen or in a single player mode against the game's artificial intelligence, although the game's popularity is derived mostly from the fast-paced player vs. player action it provides.
Liero's last release was version 1.33, released in 1999. However, the author lost the Pascal source code in a hard disk crash, and due to the lack of backup, no new "classic" versions have been released since. Despite this, and with the author's approval, the Liero community has distributed several altered (or hacked) versions of the game through the LieroCDC, and others. In 2009, "classic" Liero was officially merged with the OpenLiero project upon the release of Liero 1.34 (not to be confused with the total conversion by that name). The new versions are released by Gliptic, although Joosa Riekkinen endorses them as official.
Liero is a versatile game in terms of modification. All of its 40 weapons can be completely replaced with new ones that can be given different images and sound effects from the original set. The images of the worms themselves can be transformed into completely different characters, although their movement animations are less flexible regarding modification. The maps can be given permanent terrain other than rocks alone. Destroyable terrain can also be colored more than simply plain dirt. The AI can be modified to be harder or easier. Nearly the whole game can be converted into something entirely different, except for the main aspect having to do with slaughtering another player/AI.
Source: Wikipedia, "Liero", available under the CC-BY-SA License.