Traps, Rolling Logs, Abatis, and Defensive Trebuchets are all one shot items. Sure they may kill one or more troops but then they become useless and you have to build new ones. They do not last multiple rounds in battle nor do they last between battles. In my opinion Archer Towers are far superior because they can be used over and over.
Even if you win a fight the trap/log/abatis/treb are used up and you have to build new ones!