In the words of the unknown narrator of every Fallout game out there: "War... war never changes." Well, I tend to disagree with that on one aspect. It does change on what war game you play. Sometimes it's a tightly knit experience with clear representations for terrain and units. Other times it is a soup of components that need to be put together and thrown in an uneven field to stumble upon each other. Sometimes it's a graphical delight with crisp contours and a perfect color balance. Other times it's Warzone 2100.
Hailing back from the nether of early 3D strategy games, Warzone 2100 is kept on life support since 2008 when its code became open source. The game is relatively stable, but it desperately needs a face lift. The dark browns of the terrain and low lighting make the battlefield so bleak and uninviting that I had a difficult time forcing myself to look past it.
But getting past the ugly visuals and antiquated graphic design, Warzone 2100 is a fairly interesting beast. Unlike other RTS's, base management and resource collection are given very little focus while the unit composition and field presence become paramount to success.
The combat style in Warzone 2100 is very similar to that of the even older (but much more lovable) Total Annihilation. All weapons are projectile based, and nothing ever has a 100% hit chance. Shots might fall short of a moving target, or they may hit the hill that the target hid behind. This aspect is not at all frustrating as the number of units that are usually involved in the latter parts of a match evens out the odds.
With the exception of the Truck (worker) and your first machine gun vehicle, all unit types must be designed before they can be produced. There are three components to each unit: hull, propulsion method, and weapon/equipment. In time, you will receive new technology projects to research. These will unlock unit parts and buildings.
A player's base is not necessarily defined by production buildings with workers gathering nearby resources. Resources in Warzone 2100 are represented by oil deposits. Players construct oil rigs to claim them, after which they begin injecting resources. Claiming these oil deposits quickly is highly important as they are very cheap to set up and don't require any attention in regards to production. Therefore, bases naturally tend to be defense outposts settled around one of these rigs.
Not to say that there is no centralization in Warzone 2100. The HQ building acts like a nexus of sorts for three reasons. You can have only one, you need one to see the minimap, and it's the point where every sick unit flees to. Speaking of which, units can be set behaviors. You can set them a damage threshold, after which they will automatically break from their group and run to HQ (where hopefully you have some repair turret units waiting). You can also set them on passive, return fire, or fire at will mode. Since all weapons are a turret of some sort, units can fire in any direction even when moving.
Unit AI is not the most atrocious but it's not stellar either. Units are free to move about in order to engage targets but then go back to their last ordered position. That combined with retreating units makes for a more active battle ground.
Many high tier units are centered around artillery. Since you can build walls and stationary defenses that can rip through advancing units, long range artillery, coordinated by spotter units tends to be the hallmark of late game battles.
The campaigns bring in some extra game elements. Units and buildings persist from mission to mission, either be the game map expanding to include extra objectives and enemies or via the transport ship, which can carry a small number of units to an external location. The basic idea is that your investment keeps building on itself from level to level, giving you a sense of growth, much like in the Earth 21XX series or Homeworld. Although to be sure, Warzone 2100 is nowhere near as polished as the above-mentioned ones.
Warzone 2100 is completely free to play and can be run on Windows, Mac, Linux, and FreeBSD. Despite its age and poor looks, I think it can offer some distraction either through the campaigns or in matches against the AI or human opponents.