Complex sentence: In 1612 it was repaired carefully and on the ground plan of a pulled irregular hexagon was constructed a storeyed building with six corner bastions, insecured by a ditch and mounds.
Split into simpler sentences: In 1612 it was repaired carefully and the foundations of an irregular hexagon was laid. On top of these; a storied building with six corner bastions was constructed, secured by a ditch and mounds.