I have seen a few sources that claim House Tyrell is at a disadvantage in a 5 player game and that to balance it you can add a castle should be added at The Arbor. I've played 3 times and Tyrell never seemed to be close to victory but before I changed anything that substancially I wanted to get more experienced players thoughts.

I remember a thread regarding this and the opinion of the experienced community was that Tyrell mostly lacked for power. To mitigate this, Umbrates (you can't get more experienced than him) concluded a good choice would be to either add a crown symbol to Oldtown or to convert the castle to a stronghold. I believe he even suggested the choice should be given to the Tyrell player.

I think that the better idea to balance the 5-players game is to convert the Old Town Castele in a Stronghold. Anyway it's very difficult to win in a 5-players game for the House of Tyrell considering the other Houses and their territories, in my opinion.

It depends of the cards set you are using and if you are playing the 5 or 6 players map.

From weakest to strongest : Weak, Fair, Correct, Good, Best

If playing with 6 players map : - 3 power card set = good chances

- 4 power card set = best chances

- 5 power card set = good chances

In a 6 players game, the relation between house Tyrell, house Baratheon and house Martell is critical. If Baratheon and Martell are at peace, bye bye Tyrells. But, if Baratheon is able to hold Martells a little, then Tyrells are able to beat Martells in the long run, for the simple reason that Martells start in a relatively weak board position (few easy stronghold and cities near it and must absolutely declare war on Baratheon or Tyrell to expand behond 3 cities), altough they have killer cards to help them.

If playing with 5 players map : Well, whatever the cards set, you have correct chances if Greyjoy isnt trying to steal HighGardens. Otherwise, your chances to Fair to Weak, depending of the skills of Greyjoy and the Relative weakness (of skills) of Lannisters.

I have played this game about 30 times in the last year and a half, yet, my gaming group and myself or not fan of fan house rules (like upgrading a city to a stronghold or giving a power icon to Old town for Tyrells). If the Tyrells are, de facto, in a weaker position on a 5 players map, it is simply another gaming consideration.