Offline single player?

You can play the tower defense mode single player.

This is how I play. It gets suuuper difficult.

There is single player. Though it's especially tough to manage, since you have to defend multiple objectives that are spread rather far apart. The intensity of each wave may be affected by player count, but you obviously can't be in two places at once. The road map does list that "single player improvements" are slated for April, so hopefully it'll become more manageable. Until then, this is best played in multi player.