Вставте верно were/was и если можно объясните как найти куда что

Вставте верно were/was и если можно растолкуйте как найти куда что ставить? There was a kremlin in the centre of the town but un the eighteenth century its wall and towers ______________ (to destroy) by a great fire.In the eighteenth century the town began to grow because the road between St Petersburg and Moscow ___________(to build) and the town stood near the road.