Complex sentence: Muravyov's second assault pushed the Turks back, and he was able to take the main road and the heights over the city, but the renewed vigour of the Turkish troops took the Russians by surprise.
Split into simpler sentences: Muravyov's second assault pushed the Turks back and he was able to take the main road and the heights over the city. The renewed vigour of the Turkish troops took the Russians by surprise.