Question: what helped protect constantinople from invasion by land
[{'answer': 'Theodosian Walls', 'provenance': [{'wikipedia_id': '3592736', 'title': 'Siege of Constantinople (717–718)', 'start_paragraph_id': 2, 'start_character': 190, 'end_paragraph_id': 2, 'end_character': 206, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '3592736', 'title': 'Siege of Constantinople (717–718)', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 987, 'bleu_score': 0.9879148529393721, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 14757869569207952444}, 'section': 'Section::::Abstract.'}]}]