Written with AA in mind - Closed vs Open game:
Closed games are games where it is easy to set up a solid defensive line. Factors that can contribute to a closed game include - restrictive terrain/chokepoints, team games with larger numbers of players, and maps that are long and narrow. Strategic considerations for such a game are to occupy as much territory as possible early on. Its easy to claim territory that is unoccupied, but hard to assault territory that is defended. Generally the more territory you have, the more resources you control. As the commander is fairly tough, and can build light laser towers quickly, it is the ideal unit for initial occupation of territory. Hence moving your com towards the center of the map as quick as possible (com pushing) is a vital strategy to master for playing closed games.
In a closed game, attacks by small to medium squads of units often achieve nothing. Or worse than nothing - you have wasted metal, and the attacker die on your opponents defensive line, leaving your opponent with an economic boost if he reclaims the metal. So attacking carefully is important. A popular and effective option for battle on such a map is long range artillery. If you can build a guardian within range of the opponents line, you may be able to shell that line without taking any damage yourself. Mobile artillery while less popular can also be effective. Range isn't as great, but it can be moved to where its needed most, and can also be built quicker and cheaper than static artillery. Another tactic to consider in team play is a coordinated attack. You may be facing four players lined up on crossing with a dense defensive line across the whole map. But if four aggressive players build a significant mobile force, they may be able to concentrate it on the weakest point on the defensive line and break through. Such a break through will often be decisive, as in a closed games most players invest a lot of effort into the important defensive front line.
Also be mindful that some maps may appear to be closed, but can be opened up by specialised units. Hovercraft or some kbots can cross barriers that are impassable by normal units, allowing surprise attacks in parts of the map that may not be defended as well.
Open games are games where it is difficult to set up an effective defensive line. Open games are most likely to occur with 1 vs 1, or maybe 2 vs 2 team, on open square maps with few terrain restrictions on where units can move. In such games it is important to expand your economy as quickly as possible, while restricting your opponent from expanding.
Attacking often, with smaller squads of units is normally rewarding, as your opponent cannot effectively defend his entire territory. To build a defensive line that is strong enough to prevent this from occurring is too expensive, and will slow your expansion too much. Most attacks should take out at least a few defensive and resource structures, and require the defender to spend time moving defensive units to intercept, which slow his expansion. Expect that when you are attacked that you will take losses, and don't over defend. A few lighter defenses scattered around to slow the enemy down, and tip the odds in your favour when your mobile units intercept is all that is required.
Artillery is less effective as it is either static or slow, and there are no key points to attack. If you build a guardian, you may find that all you have achieved is shell a few cheap defenses, and claim a small amount of extra territory that is not strategically significant, after expanding a strategically significant amount of resources. Com pushing can be dangerous, as it is easy for the opponent to move a party of raiders around the defensive line the com is attempting to set up. They may then have uncontested access to destroy your factory and key resource structures. The com is often more effective guarding the factory as a last line of defense against raiders, and to help you to quickly pump out large numbers of raiders and construction units to attempt to out expand and out raid your opponent.