Get a computer and automate it. You'd have to know some programming (well, quite a bit actually) but it could be done.

Or you could just use the simple method of deciding what happens with all the NPCs and only rolling where it concerns the PCs.

They could hold the top of the wall in an out of the way place. This way they could rain arrows, burning oil, sling stones, etc. down on the orc horde without too much danger. When the seige ladders finally went up, they could push them off, and if an orc did manage to get to the top, the fighter could drop his bow and fight that one orc. They would see the gate below falter, and the fighting begin in the main keep, but as long as they stay at bow range, they'll probably be fine. Have a few hails of orcish arrows pepper the top of the wall (perhaps even grazing or striking a pc or nearby NPC) and you'll make it real for them. Also remember that orcs are (or were) challenge rating 1/3, where a pc party is worth at least 3 orcs.

How things go will depend in part on the willingness of the players to go along and risk their newly created charaters lives in defense of the town. How you deal with this is up to you. If they are reluctant you could have the mayor or whoever offer a reward, have a wounded scout die in the hands of the party Paladin or other duty-bound type so they'll feel obligated to avenge the poor scout's sacrifice in bringing news of the oncoming horde, etc. Once you have them set to help, I might suggest having them man a sally port. Basically a small gate through which cavalry or other harrassing forces might be sent to break the enemy's lines and say get a messenger out to warn other towns of the orc horde. Naturally, it will take several attempts to succeed at this. The party could be put in charge of the gate's defense as orcs try and rush in each time it is opened to let a sally group out or survivors in. In this way you can meter things such that the party has time between fights to recuperate as best they can and prepare for the next fight. If a fight does start to go badly, you can always have the sally group forced back through the gate due to "too many orcs in the area" or an expendable troop of soldiers come to the party's rescue. I figure you can get at least one good tense session out of this, give the party some decent xp and other rewards, and make the players feel a bit more involved in the greater events of the land.

Let's say that the orcs need a guide to get them through a particularly tough area on their way to the town. There isn't an orc with the necessary abilities. So they kidnap a moderate-level druid and (a) charm her with a powerful potion, (b) threaten her animal companion / child, or (c) use psionics on her until she's insane and unable to tell what she's doing.

The party needs to rescue the druid, either through (a) recognizing the situation and dispelling the charm, or (b) find the hostage and free it, or (c) find a healing potion and get her to drink it.

You could have your characters do a bit of reconnaissance(sp?), maybe some hit and run tactics, or you can send them a little behind the lines to attack a poorly guarded supply depot. Maybe have them attempt to break out and go for reinforcements with a squad of orc scouts or horsemen hot on their heels. From a roleplaying side, you can have them help with the town's defenses, rally and provide training to the local militia, or act as liaisons between different power groups that are defending the area.

Have the PCs be assigned to help evacuate non-combatants from one section of the town into the central keep. Have there be a breach in that section and have the PCs lead a retreat through the city. I find a chase more thrilling than a stand-off, and this forces the party to keep on the move and can force some tactical thinking -- especially if some orcs come out ahead of them in the chase and they need to punch through a small party before the rest of the overwhelming horde reaches them.

Have excessive amounts of fire, roll a bunch of attack rolls, but have far flung arrows or thrown axes miss regardless of the die roll to give them the sense that it's overwhelming.

Maybe give them an option to cause some shift in the in-town terrain that will prevent the breach from being useful. Perhaps they're in a section of town that is cut off from the rest by a stream running through the town, and a wooden bridge is what connects it to the rest of the town (and, as a result, the keep). Have some kind of setup so that they can cause the bridge to collapse either via direct action (light lighting it on fire or damaging the supports) or by something indirect (like lighting a barrel of blackpowder or alchemists fire and rolling it towards the supports of the bridge). The current is so swift that the orcs cannot cross the water, armored or not. So either the breach becomes insignificant and they're forced head back outside the walls and try and come in another way, some try and cross and are swept under/away (reducing their numbers), or they waste time bringing in something that will permit them to cross, allowing the defenders to reallocate forces.

If they don't opt for that and you don't want it to be a slaughter, have them reach a sort of gated secruity checkpoint that limits the orcs to a specific area, forcing them to breach THIS game, that gives them another chokepoint to protect.

That way they can still engage in active defence, but it picks up with a chase.

I don't know, I like chases. :P

I havnt read the other responses, but where IS this keep town? Is it like the place in Two Towers, nestled against a mountain?

Or is it out on a great plane?

If its in the mountains the PC's can go out into the crags and roll giant boulders down on the orcs.

If its in the planes they can maybe set up various little traps that slow down or help break up normally well organized units.

Perhaps they can seek out a druid that protects the local area and through RP convince him to use his powers to divert a river to flood the plane or valley the army is coming from! At the best the army gets flooded out or swept away, at the least their movements are slowed and they become disorganized.

Maybe they can run to another keep to get help and perhaps bring a second front against the orcs.

What kind of characters is the 1st lvl party made up of?

For example, is it heavy in fighter types? Go for more of the 'guard the gate' and other hand to hand combat missions as was recommended by others earlier. Are they druids and rangers and the like? Maybe send them outside the gates skimming the woods for scouts sent ahead by the orcs.