How many humans to hold down your 5km border?
Low-tech? You're pretty much screwed, especially if there's a numerical superiority of attackers (3-to-1), or if you're spreading your defenders out (ie: attackers can put together a disproportionate force at one point vs. defenders at that point). The larger the border you try to hold down, the more dispersed your defenders are. If you're leaving one guy holding down 50' of border - it'll only take 3-4 human equivalent attackers (minus however effective your barrier is) to overwhelm him, and pierce your defenses, then roll up the flanks of your other defenders. If you can see them coming (low-tech means you're screwed for night attacks), and can move quickly, you can have your defenders grouped up but spread out a bit more (well, not a lot more than 100'), so they can come to each other's aid. Beware of feints to draw in surrounding defenders leaving holes in your defenses. The larger your defending group, the bigger the attacking group will need to be. Also, figure in your advantage in ranged weapons and armor. Unless your monsters are pretty tough.
If you want it realistic, have your humans move somewhere with better defenses. An island, a peninsula, or land between two rivers, or as Pavel suggests, a cave. Or build a castle (and pack in the defenders).
A barricade isn't going to be effective against anything that can climb, dig or leap (for reasonable barriers - if you can build them 20' high and make them smooth - maybe you've got a chance). Most predators are going to be capable of scaling 10' and 12' fences. I've seen coyotes jump over 8-10' fences. Once you start trying to build fences higher than 10', especially of anything substantial, you're going to have to start building scaffolding and putting it up and taking it down. Block and tackle rigs, etc. That's not going to be fast. Heck, logging your forest isn't going to be fast either (without chainsaws, steam-donkeys or bulldozers/tractors). Nor is evening up your trees, so you don't leave climbable outsides, or chinks inbetween them. You're going to have to drag your trees from further in the forest to your perimeter line. At around a 1' in diameter, figure you're going to have 20 to 40' lengths, and need ~12 per 10' in height (trees aren't parallel). ~5468 trees you're going to need to cut down for a 10' high fence. ~20m for an experienced ax-man per tree, assuming nice iron/steel axes are available - stone axes means a ridiculous amount of time more. How many axes do you have? Sharpening stones for each ax? New ax handles for the inevitable breakage? Around 30m to an hour for an unskilled worker. After which they're going to have sore hands, be winded, and the next one will take longer. You're also going to max out at somewhere around 6-12 hrs of (useful) work per day, of heavy manual labor, for unskilled people. You've still got to lop the limbs off, and drag it to your fence line, and/or lift it into position. How're you attaching these trees? I hope you've got a lot of rope. It might be possible to get many trees that're close enough that you can wedge them between alternating trunks. That will take a fair amount of surveying and planning. Btw, the number of trees given is for a perfectly straight 5km - if you vary your line at all (in order to use existing trees as uprights for example), it's probably not going to be straight - so be sure to add more work. You could also use some felled trees as posts, and dig out a nice hole near one upright tree and put a post in, keeping your horizontals inbetween the two. Still going to need a fair bit of rope to be putting up each section. If you don't have that much heavy duty rope / chain available, you're only going to be able to put up each section one at at time. You've got 546 sections to do. Let's WAG it at 10 hours for a 10' section (even higher gets an even longer time) - that's 227.5 days if you've only got one rope-set. You'd be better off spending some of your time making rope / chains for starters if you don't have enough.
Also, remember that logging, especially old-school logging, is one of the most deadly professions. And even more-so for amateurs and for people who're working under stress; tired and fearful. They tend to make even more mistakes. Expect to kill a number of people in putting up these defenses.
Old school trenching (shovels, etc) is even more labor intensive, backbreaking, and not very effective at keeping animals that can climb / leap out. It'll work on horse-like monsters. But not so great at anything else.
If you've got the proper tools and horses, you can make some stuff which will move ground faster than shoveling. Might be worth your time to spend it building that equipment, then get working on trenching. But that's also going to be determined by your soil. Got clay? Some clay is next to immovable. As is bedrock. They even have trouble dynamiting that clay out here. It just laughs at jack-hammers. I can only imagine trying to go after it with a pick and shovel.
Are your monsters afraid of water? Would a moat work? Redirecting water to cut a trench would be much more effective, especially if you've got height differential to work with. You only have to survey and cut a nice starter trench, then start dumping the water through. 5km may be a little long to expect the height differential to continue working, however. I know you can easily get 500' to clear with a reasonable head of water.
