Complex sentence: During this period, the citadel was only once taken by force through mining of one of its walls in 1239, but this occurred when the citadel's garrison had been reduced to below the number needed to defend a castle of that size.
Split into simpler sentences: During this period, the citadel was only once taken by force--through mining of one of its walls--in 1239. This occurred when the citadel's garrison had been reduced to below the number needed to defend a castle of that size.