Misdirection...trickery...defense in depth...
Let's look at this from the Orc standpoint...does the Orc leader "know" he is only facing the NPCs or is he unsure about what he is facing? I agree with KC...that is a lot of ground to cover with a small force. Also, if I am expecting to be assaulted by a large(r) force, my tactics are much different than if I am trying to keep a small infiltration unit out.
If my orders are "nothing gets through", here is what I do...
1) Defense in Depth - My 50-orc unit cannot cover the entire frontage, so I place a series of small obstacles to slow any enemy advance, determine their direction of attack, etc. Caltrops, pits, snares and bear traps are my friends here. Smaller, prepared fighting positions with ample stocks of missiles, interlocking ballista fire. My orders to my troops are to abandon forward fighting positions when pressed - trading space for time and whittling away at the enemy until they fall back to the "final protective position".
2) If I have some magic support - I use illusionary visual and auditory magic to make it seem like I have many more troops than I actually do. Straw dummies with a minor illusion, countermarching a squad around the position with 3-4 different colored tabards on, twice as many campfires as normal, etc so that a casual recon by the enemy might lead them to think I have 200+ troops. My caster is also tasked with using illusion to enhance the size of my counter-attack force (see below).
3) Anti-Infiltration Patrols - I would probably clear several 30'-40' wide swaths perpendicular to any expected infiltration route and spread flour, lime or another high contrast material so that a patrol could quickly see (visually) if creatures had passed through a defense zone and what direction they headed off in. A couple of wolf/warg and orc ranger teams patrol these zones.
4) If a breakthough occurs, my troops don't DIP (die in place), they break contact and flee, at all possible speed, to our rally point, where I have my counter-attack force (made up of big hitters - the ogres, trolls and an elite squad). Ideally, I fix the attackers on the final protective barrier and then hit the enemy from the flank or rear with my counterattack force.
5) Trap Door Spider - One of my favorite nasty tricks. Certain orcs (or a whole squad) have orders to "take a dive" as soon as they are hit or take damage. If they compatriots are fleeing and the enemy presses the attack, they aren't likely to check and ensure everyone is dead (they could even use bladders of animal blood to make it look more real). Once the battle has washed over them, they follow the battle and strike the enemy from behind once the battle at the final barrier is joined. An equally effective tactic is to have an elite strike team go to ground in a well-hidden location close (large pit) - but in front of - the final protective defense line. When the enemy engages along the final barrier, the elite team (troll, an ogre and a dozen orcs) burst from the ground and hit the enemy rear (mages, clerics, etc).
The major problem with a "static defense", as other have noted, is that it is very difficult to keep out a small infiltration team.
Good luck!
~ OO