A roundhouse is completed when all stations in a category or class have been played in consecutive moves by a player. Each roundhouse has its own particular effect - these are listed in Appendix (viii) of the annual rule update, as the effects are modified from time to time. It is vital that the ruleset governing each game is specified so the correct effects can be used. Default effects tend to be the 1963 set which have some unusual characteristics to catch the unwary. The strongest roundhouse is the South, in which all stations South of the river are played. Any player who completes one is virtually certain to win the game.
The roundhouse was a domain of the more defensive players until the famous 1968 World MC championships. In the quarter-final (Trellis vs. Sandage) the inimitable Trellis opened up a whole new array of possibilities by applying field theory to the roundhouse.
Trellis was able to constrain the Purple stations and in effect play them simultaneously. As order is irrelevent in a roundhouse, as long as the stations are constrained, one can play any one station and claim the roundhouse's effect. Trellis again confirmed her mastery of the game, and such a maneouvre is now called the Trellis Roundhouse.