In the forum's question of the week, the developers have revealed that base structures will have durability meters, as buildings just don't "break apart" the minute they're hit by something. However, these durability meters will slowly decline over time, representing wear and tear from the elements. This will require guilds to make sure that they upkeep their buildings, much like a real mechanic needs to check up on a building's machinery every so often. This "durability decrease" is currently set to a very infrequent amount, so busy guilds don't have to be repairing buildings constantly.
Attackers will also have combat options, but these will come in the form of mechs and temporary structures that can be placed on the map. These options have nowhere near the permanence of what defenders can produce, and that makes a large amount of sense to us, at least.