Trap doors
Trap doors are the first featured chase-breakers used in the Assassin's Creed: Brotherhood multiplayer. They consist of a pair of gates that close once a player runs through them, effectively stopping any other player or enemy from following their path. After a few seconds, however, they reopen, allowing other players to run through them. If a player runs at them while they are still closed, then the player will simply start a free running sequence.
Other then the players, NPC`s will not go into a passage of trap doors so it is a good way to spot other players. A player will also not activate a trap door to close if they walk by it in low profile.
Occasionally, if a player is sufficiently close enough to their target they will still be able to assassinate them just as they enter the gates, with the doors closing behind both players.