Complex sentence: The single door is surmounted by a triangular pediment and the arch's external voussures are decorated with carvings of acanthus leaves whilst above the door is an ornate finial above which is a "hermine passant" (a "stoat passing" in heraldic terms).
Split into simpler sentences: The single door is surmounted by a triangular pediment and the arch's external voussures are decorated with acanthus leaves. Above the door is an ornate finial above which is a "hermine passant" (a "stoat passing" in heraldic terms).