The next morning a bridge was thrown over the Agueda at Marialva, six miles below Ciudad, but the investment was delayed, owing to the slowness and insufficiency of the transport. Ciudad Rodrigo was but a third-class fortress, and could have been captured by the process of a regular siege with comparatively slight loss to the besiegers. Wellington knew, however, that he could not afford the time for a regular siege. Long before the approaches could have been made, and the breaches effected according to rule, the French marshals would have been up with overwhelming forces. Beginning the investment on the 7th, Wellington determined that it must be taken at all costs in twenty-four days, the last day of the month being the very earliest date at which, according to his calculations, any considerable body of French could come up to its relief. Ciudad lies on rising ground on the bank of the Agueda. The fortifications were fairly strong, and being protected by a very high glacis, it was difficult to effect a breach in them. The glacis is the smooth ground outside the ditch. In well-constructed works the walls of the fortification rise but very little above the ground beyond, from which they are separated by a broad and deep ditch. Thus the ground beyond the ditch, that is, the glacis, covers the walls from the shot of a besieger, and renders it extremely difficult to reach them. In the case of Ciudad, however, there were outside the place two elevated plateaux, called the great and small Teson: Guns placed on these could look down upon Ciudad, and could therefore easily breach the walls. These, then, were the spots from which Wellington determined to make the attack. The French, however, were aware of the importance of the position, and had erected on the higher Teson an inclosed and palisadoed redoubt, mounting two guns and a howitzer. A great difficulty attending the operation was that there were neither fuel nor shelter to be obtained on the right bank of the river, and the weather set in very cold, with frost and snow, at the beginning of the siege. Hence the troops had to be encamped on the left bank, and each division, as its turn came, to occupy the trenches for twenty-four hours, took cooked provisions with it, and waded across the Agueda.