There are a few different ways 2 people can play Settlers of Catan, here a few:
Idea by Thomas A. Stobie
These rules adapt the original rules for use with just two players. These can be used with Settlers of Catan or Seafarers of Catan.
Each Player uses two colors.
Trading with the other player is not used. Trade with the bank only.
20 Victory Points are required to win (higher totals can be used for longer games).
Increase the threshold from 7 to 9 where discarding on a "7" is required.
Idea by Ethan Sprang
Each player is limited to only one set of pieces.
Players play to 15 points.
Players may not trade resources with each other - trades are only allowed at ports.
Place three settlements with roads in initial placements phase. Player "A" that rolled highest goes first, placing one. Then "B" places 2. Then "A" places 2. Then B places last.M/
Each player receives starting resource cards for their last two settlements.
Player "A" begins, and has the first turn.
Play starts. The robber cannot emerge for 2 turns. If a seven is rolled, roll again.
Each player rolls twice on their turn for resources.
If a seven is rolled (after turn 2), the Robber is played as normal (except a player discards half if they have more than EIGHT cards).
If a seven is rolled on both of a player's rolls, on the second seven the player can elect to not move the robber a second time. Also, the player can elect to take any two free resources from the bank as opposed to taking a second resource from the other player.
Idea by Klaus Teuber
Many people enjoy playing "the settlers of Catan" as a game for 3-4 players (and with the appropriate expansions for up to 6). But ever since the initial appearance of the game there was a strong desire for the possibility of a satisfactory Settlers game which could be played with only two players (which is apparent from the fact that after the 5-6 expansion the very next one was a Catan card game, basically a Settlers version for 2 players).
And even though the Catan card game has been very well accepted - it nevertheless plays rather differently than the boardgame it was based on.
Why can't the board play be played satisfactory with only 2 players? There are two big reasons for this. The first one is that the island is simply too large for two.
There are too many building and spreading possibilities and one can always easily evade the opponnent. As a consequence the feeling of competition diminishes as one player can develop more or less indepentently of the other.
The other reason is that one of the more substantial aspects of the "Settlers of Catan" - the trade with the fellow players - simply fails to deliver in a 2-player game.
Two opponnents will usually have no incentive to trade under normal circumstances - if they both gain the same, then no player really gets closer to victory in relative terms. On the other hand, if one player gets more then the other, the trade will not happen. Either way, trading is rather pointless.
For this dilemma, there is now finally a solution. This document will describe a successful variant to basic Settlers game which works quite well with only 2 players.
Cut out the commercial chips (you can find them at the end of this document).
These commercial chips will be used by both players throughout the game.
The game is prepared according to the normal rules. The two figure sets (which were not selected by the players) are taken by two imaginary neutral players, and are laid out beside the board. The commercial chips are laid out likewise. Each player initially receives 5 commercial chips.
First off, two settlements (without a road!) of the neutral players should be placed in accordance with the illustration below. Afterwards both players create their two starting settlements with roads according to the normal establishment rule.
The Flow of Play
The game is played in accordance with the 3-4 player rules, but minding following exceptions.
1. The Dice Throwing
On his turn, player throws the dice two times instead of one (neutrals never throw dice!). The two numbers must differ. If the same number is thrown, the dice should be thrown one more time. If a "7" is rolled in either of two throws, the player moves the Robber and the usual actions are taken.
2. Neutral Building
Whenever a player chooses to build a road/settlement, he also builds one for one of the neutral players (free of charge). The player freely chooses for which neutral he will build.
The player MUST build the same type of neutral building as the one he built for himself (if he built a settlement, he must build a neutral settlement). The only exception is when no settlement can be built for either of two neutrals because of the placement rules - a road can then be built instead.
Building a city or buying a development card does not affect neutral players in any way. Neutral settlements do not receive resources. However, it is possible for a neutral player to receive "The longest road" card.
3. Spending Commercial Chips
On his turn, a (non-neutral) player can choose to perform one of the actions described below. Performing an action uses up one commercial chip for the player with less victory points, or two if the player is in the lead.
If both players have the same number of victory points at the time of spending, the action costs two coins. The actions are: Obligatory trade: The player can pull two resource cards from fellow player and gives him two resources in return.1 Chase away robber: The player can put the robber back to the desert.
4. Acquiring Commercial Chips
For every settlement placed near the desert, a player receives two chips (this also works in the establishment phase).
For every settlement placed on the coast, a player receives one chip (this also works in the establishment phase).
Also, the player can earn two chips for disbanding a soldier. When the player chooses to disband a soldier, he discards one soldier card previously played.
The soldier can be disbanded from the reserve at any time on player's turn, and the player can discard as many soldiers on his turn as he/she. The biggest army should then be recounted. If the player has less then three soldiers, he loses the biggest army card.