Complex sentence: Although the Fortress of Louisbourg's construction and layout was acknowledged as having superior seaward defences, a series of low rises behind them provided attackers places to erect siege batteries.
Split into simpler sentences: Although the Fortress of Louisbourg's construction and layout was acknowledged as having superior seaward defences, a series of low rises behind them made the Fortress vulnerable to a land-attack. The low rises provided attackers places to erect siege batteries.