Strategy game that really does it own thing, A.I. sucks after a while, but well its an older game. Would recommend playing this with people after you get to know the game.
A bit more in depth:
In Perimeter the most important things is your main building, to which your first energy nodes have to be connected, and energy. you build an energy network by building energy nodes, which will collect energy(only resource in the game) for you and allows you to build other buildings in an area around that node. Every energy node has to be connected to your main building or another energy node that is connected to your network. If a piece of your base becomes unconnected it loses power and all buildings and nodes in that area become unpowered(neutral) this allows you to steal portions of your eachother's base. Energy management is key to this game, because almost every action will cost somewhere between a bit and alot of energy. Luckily you can also create a rainy day fund by building energy storing buildings.
Terraforming is the next important thing in the game. Sadly you can't build your own worlds. But if you want to build somewhere in Perimeter you need to make that area to "zero layer" (flat) ground. The closer the non-terraformed land is to zero-layer the faster it goes. You also can't extract energy from non-terraformed ground. You get 5 units that can terraform OR build. which gives you the choice for faster terraforming or building. Terraformed land can be destroyed again by any sort of explosive which leaves a crater that will slowly destroy any buildings on the now non-terraformed land, which could cause your building to blow up and leave another crater, which could cause buildings around it to be destroyed (and a chain reaction if you built all your buildings next to eachother).
In Perimeter you don't just build units and attack. The focus lies more in attack with your base, by building your network towards eachother and start firing some howitzer turrets to destroys eachother's base and land. attacking weak spots of the enemy with units or just sending wave after wave on a spot you know is not going to hold forever. You also get some special units, like an unit that can steal energy from the other, one that makes your units (except the unit himself) invisible or a few that can ruin terraformed land in a few ways. Units fill more a support role or raiding role in this game for me,
Turrets are your main way of attacking/defending. and the two factions each get a special turret (bit superweapon-ish but not OP, also costs alot of energy to charge)
Another important thing about units is they can transform into other units. How does this work? you have 3 basic units in the game: The soldier, the officer and the engineer. all other units are composed of a combination of these 3 units, example: 6 soldiers, 3 officers, 0 engineers. This allows your units to transform on the fly if the need arises(altough it leaves your units exposed while the transformation happens). Or change the units later into something else when you have better laboratories set up.
If all hells breaks loose and an area needs just that few extra seconds for your turrets to finish building and your force to arrive, you can throw up your Perimeter shield. The shield is indestructable but it drains your energy. Every energy node has the ability to throw up the Perimeter shield around the area of effect of the node and it will protect all buildings in that area of effect. Connected nodes will connect their shields. You can activate the shield individually per node or press Enter to activate all.
You win the game by destroying or capturing your enemy's main building.
NOTE: it took me a while to know how to capture a base. Some campaign missions ask for it and they don't tell you how to do it ingame. you have to build an energy node right next to the enemy's base and let his main building take off in the air. (destroy the ground under it so it takes damage) It's a bit annoying but you'll find out :P)
Awesome Game for being something different even after 10 years. 10/10