The tower in particular was of enormous size, becoming known as the 'accursed tower' afterwards. In burning down, fire spread to the walls and burned for 24 hours before a section of the walls collapsed (according to William of Tyre). The breach was small but enough to persuade the citizens to surrender, having already endured privation for several months. There may have been a weak-point in the structure of the walls anyway: having worked on restoring old buildings myself, getting the right mix of cement, lime and clay in excessive heat is difficult even today. Stone blocks may easily have been fixed together with mortar that cracked/powdered when fired. The walls of 1153 had been built on top of the ruins of previous walls and did not have integral foundations. Structural weakness at the base of the wall is probably only to be expected.
As to suggestions for campaigns, you have hit something of a jackpot, if I recall correctly. In the era of Baldwin III, you not only have conflict with the Saracens and Fatmid Egypt, but a civil war effectively against his mother, Melissende. The military orders were the only standing army in Christian service (apart from palace guards and the like). I am not up to date with the latest research in this area, but I suspect that even in times of 'peace', crusader-barons settled disputes with each other by the sword.