The impregnable character of the _enceinte_ was thus ensured. But further skill was necessary to defend the entrances of the camp. Of these there was usually more than one, and these were necessarily formed by cuttings through the banks. As in the case of Maiden Castle, the path of entry could be converted into a labyrinth by multiplying the banks and ditches. Every inch of this circuitous path is guarded by tall ramparts: there are seven or eight points in its course at which fatal error was possible, and one at any rate where an attacking army could hardly fail to rush securely upon destruction. The eastern entrance is so guarded by transverse banks that the path is almost equally difficult to find. It was seldom, however, that entrances were so elaborately protected. At Old Sarum the western entrance is covered by a semicircular outwork, on the flanks of which are the two inlets to the passage through the outer _vallum_. On the east side the entrance is through a narrow passage which runs for some distance between the parapet of the outer ditch and an outwork, and is at right angles to the actual passage through the bank. The most common method of defending the entrance was to make a diagonal path, usually from right to left, through the banks. Each bank would thus overlap the next: the summits above the path would be broadened out into platforms, capable of occupation by bodies of defenders; and the right flank of the enemy, unprotected by shields, would be exposed to their missiles. The entrance, however, might be substantially protected by giving an inward curve to the inner bank on each side of the path; and this is a plan very frequently employed.[3] Where outworks were specially constructed, their form differs considerably: we have seen them employed in two ways at Old Sarum—as a kind of horn-work thrown out in front of a passage, and as a spur projecting at right angles in front of an entrance. In both cases an absolutely straight approach is precluded. Occasionally, where an approximately direct approach was permitted, it is guarded on one side by a spur thrown out at right angles to the bank. At Blackbury castle, an early earthwork in south Devon, the main entrance has a straight approach guarded on either side by a triangular outwork, which is formed by a most ingenious arrangement of banks and by prolongations of the main ditch (7). At the actual entrance the main bank is curved outwards, with broad platforms at the top. The hollow interiors of the outworks might serve as guard-houses, or the attacking force could be driven into them from the gateway of the fort, and penned in a position from which escape was impossible. Sometimes hollows were made in the bank or in a projecting outwork near the gateway to serve as guard-houses. On either side of the main entrance, the bank was often slightly raised. At least one instance is known in which the main gate was concealed by making a break in another part of the rampart, and