 Complete the following phrase:  In the first half of the 18th century, the Russian Empire constructed the Irtysh line, a series of forty-six forts and ninety-six redoubts, including Omsk 1716, Semipalatinsk 1718, Pavlodar 1720, Orenburg 1743 and Petropavlovsk 1752, to prevent Kazakh and Oirat raids into Russian territory. In the late 18th century the Kazakhs took advantage of 