Complex sentence: The forts of the first construction stage were built accordingly to typical design of Russian brick fort of 1879, therefore first seven forts have similar construction with the only difference in inned planing, integration into surrounding relief, later renovations and some constructional elements.
Split into simpler sentences: The forts of the first construction stage were built according to the typical design of Russian brick forts of 1879. Therefore, the first seven forts had similar designs with the only difference being the inned planing, integration into surrounding relief, later renovations and some constructional elements.