Currently you can build in a "creative mode" only by using console commands to access a free build command where you can also play in god or ghost mode so mobs leave you alone. Most building fans generally have a separate seed on which they will build and "live" in and another seed to farm and explore in just so they avoid the inevitable PVE raids the game triggers as you play.

In terms of combat, you will progress through different levels, each getting slightly more difficult and does the usual unlocking of higher-spec materials to craft from. This brings me back to the point about the raids above because as you beat each boss, harder raids will happen and they like to attack you at your base... which is why build fans will build on a seed where no bosses have been beaten and thus no raids triggered.

The game isn't especially hard but can be a little tedious and grindy at times, particularly if you're playing solo. It's probably the most fun with one or two other people with whom you can play through together at the same pace as all progression is tied to the world seed you're on and NOT the individual player.

There is an update coming relatively soon which will allow you to customise your game play without having to go into the console which is something I am looking forward to, personally.

All in all, as an EA game it's pretty darn good and very playable. There's also loads of mods available if that's your schtick as well. It was worth the $20 at first launch and still has solid value now with the updates they've done since then.