It consisted of three lines of batteries and intrenchments.
The second was the most formidable; but the first was so strong, also, that Wellington determined to defend this, instead of falling back to the stronger line.

At the foot of the line of mountains on which the army was posted, stretching from the Tagus to the sea, ran two streams; the Zandre, a deep river, which extended nearly halfway along the twenty-nine miles of lines, covered the left of the position; while a stream running into the Tagus protected the right.

The centre, therefore, was almost the only part at which the line could be attacked with any chance of success; and this was defended by such tremendous fortifications as to be almost impregnable.
Massena, who had only heard vague rumours of the existence of these fortifications, four days before, was astounded at the unexpected obstacle which barred his way.

The British troops, as soon as they arrived, were set to work to strengthen the intrenchments.

Trees were felled, and every accessible point was covered by formidable abattis.