1. When using the Tower expansion, can a follower on a castle be captured by a tower if only one of the tiles that the castle is on, is within the reach of the tower?
Since the P&D and Tower expansions existed prior to the BC&B, and it was not stated otherwise in the instructions for BC&B, we play that any mechanic that would remove a follower from a tile will remove a follower from a castle if either of the two tiles is in range. We play the castle is not a safehouse. For us the Plague will also remove a knight in a castle if it lands on either of the tiles.
2. If you finish the last neighbouring feature when adding a tile and creating a castle, will the castle score these points?
The rules state that the Castle is completed when the
first or next feature is
later scored. We take this to mean a feature that is scored on a later tile play. A feature completed as the tile is played is actually completed prior to the existence of the castle - therefore it cannot trigger the castle to score.
3. When using the Siege or Cathars expansions, will the castle score only half the points when scoring the points of a city under siege?
Agreed, the castle receives the same
reduced score.
4.From the CAR: "When converting to a castle, it does not matter whether the small city was already occupied by a follower in the first half of the city, or by the active player deploying a follower to the second half during his or her turn." Does this mean that another player than the active player can occupy the first half of the city, and the active player can still convert to a castle?
The rule states: "When a player forms a town - consisting of only two semi-circular city segments, the
owner of the town can decide whether he completes it normally and scores 4 points or converts it into a castle." So, another player may complete your two-tile city but you get to choose if you want to convert it to a castle that you own. The other player has no standing concerning your city. There is a clarification given later however that addresses how to proceed if the city was unoccupied to begin with - that is, he may claim the town and convert it to a castle on that play.
With regard to the magic gate, the rule states that a castle is completed when a feature in the vicinity is completed. We reason that if any feature in the vicinity of the castle is complete at the time it is vacated, the castle must be considered complete and not eligible to be entered by the magic gate. It is too much to try to remember which feature was completed prior to the castle being formed and what existed at the time it was vacated.
With all of that said, all of our beliefs are based on the English translation rules. I do not know how closely they match the original intent of HiG.
Hope that helps.