Read this. IMHO, it's one of the most well-defined, reasonable, & extensive breakdowns of the Invasion gametype. Many points in this thread are included in this guide, including Invasion's focus on infantry combat, the supportive (not dominating) role of vehicles, & the organization of fireteams for strategic gameplay. Frequent mistakes that seem to be made when making Invasion maps tend to be (in no particular order) A) placing WAY too many vehicles on the map, B) placing vehicles, power weapons, & the like across Tiers in such a way that the feeling of escalation is lost, & C) creating a WAY too huge map. I could go on & on about how each Tier needs to be connected to the next, & how one area of a map should not be discarded after a Tier ends, but it would save both of us time if I just keep my mouth shut & let you read the guide. I hope it is useful to your endeavors!