You'll need to buy the base game to play the basic scenario (or variants like "Ore for Wool" and "Harbormaster" that are included in the base game) with a randomized board in all game modes. Your fellow players in multiplayer games will need the base game for this as well. The base game and expansions are 500 Catan Gold each. If you still have your registration bonus of 100 Catan Gold, you'll just need the 400 Catan Gold purchase for 3,99.
We currently do not offer a larger board (5/6 player variant) for the base game. However, if you like larger scenarios, there are various included in the Seafarers expansion, e.g. "Greater Catan".
@JazzAnn Please note: The cities and knights expansion ( like in the physical game box) does "only" include the additional rule set. If you want to play the cities and knights rule set on more different scenarios, the Seafarers expansions offer scenarios you can also play with the cities and knights ruleset.
Tip: You can unlock the seafarers expansion using a scroll (for 24 hours), to test what scenarios you can play using the cities and knights rule set and if you want to invest the coin for the Seafarers expansion.
@katrae also who is I have no idea why they have gone with the 3D feel. To me the main benefit of playing a digital version of a board game is that you don't have to mess around with physical pieces, like in Monopoly where you are trying to fish around under the board for those $1 notes.
Also I find it annoying not to be able to see the board whilst trading. Maybe that person wants to build where I want to build etc.