Complex sentence: The most ancient part dates probably from pre-existing Roman and medieval fortifications; the castle in its current form was begun in 1433 or 1438 by Sigismondo Pandolfo Malatesta.
Split into simpler sentences: The most ancient part dates probably from pre-existing Roman and medieval fortifications. The castle in its current form was begun in 1433 or 1438 by Sigismondo Pandolfo Malatesta.