Taifas often hired Christian mercenaries to fight neighbouring realms (both Christian and Muslim). The most dynamic "taifa", which conquered most of its neighbours before the Almoravid invasion, was Seville. Zaragoza was also very powerful and expansive, but was kept in check by the neighbour Christian states of the Pyrenees. Zaragoza, Toledo and Badajoz had previously been the border military districts of the Caliphate.