Step 1:
Place Redstone under each block that will have a pressure plate on it.
The Blue Wool represent ground level. That's where the pressure plates will go. I didn't make mine 70 blocks long because I got lazy and the design works the same for any length. I'll cover width limitations at the end.
Step 2:
Cover the area with Pressure Plates. If mob detection is your goal, use Stone Pressure Plates. If you want to detect any entity, use Wooden Pressure Plates.
Step 3:
Place repeaters along both long edges of your area. After each Repeater, place another Redstone.
As you can see, the Creeper activates the Redstone under the Pressure Plates causing the Repeaters within 15 blocks to activate. Were he to go too far from these repeaters, the repeaters on the opposite side would activate.
Step 4:
To ensure your output signal can make it to wherever you want it, you're going to need a few more repeaters. To be safe, I recommend putting an additional repeater every 15 blocks as shown below.
This will ensure that there is always a repeater within 15 blocks of the original signal. These repeaters are where you want to take your output. Wire them together as you see fit, keeping in mind that distances over 15 blocks will require more repeaters.
Limitations
Width: Since Redstone signal can travel 15 blocks from a power source, 30 blocks wide is exactly the limit on this design. If you want wider than that, your going to have to deal with deadzones.
Best Answer
There are different kind of secret triggers in minecraft:
Entity movement
Living entities like animals and monsters can only move if a player is in a 32 block radius, this radius is a spheroidal box around the cow. By placing the a cow in a place where pistons keep pushing the cow back you can make a proximity detector.
This can be seen at:
Tripwire
If properly applied, tripwires are hard to see, this works the best if the tripwire is placed around the eye level in buildings, or at the feet in the outside world.
Hidden pressure place
Its also possible to make a pressure plate completely hidden, for example, stone pressure plates can be nicely hidden in a hallway that goes down, the following image demonstrates this:
Did you see the pressure plate at the end of the stair?
Block update detector based detection
Some blocks have a special property that they update when the player walks over them, for example, redstone ore blocks start glowing. This can be detected with a BUD to deliver a redstone signal. A simple implementation this looks like:
The item through a corner technique
Items can be pickup through the corners of the blocks, by making your player pass past such corner, you can detect it using a wooden pressure plate in combination with a torch.
The casual player technique
What if you didn't need to make a hidden trap? You could add a couple of iron doors in combination with buttons to your building, this way the player going through it will thrust the button to continue, and keeps pressing them to open the next door.
The trapped chest
By placing a large number of loot crates through your building, player will get blind opening chests, they will just open every chest to get a little loot. By making the trap have a trapped chest in combination with some delay, the player will suspect nothing.
The minecart in a corner
Like items, you can also interact with minecarts in corners, by combining this with a powered rail and a detector rail, you have a accurate corner touch detector.
Command blocks
If you are making a custom map, you can also use command blocks for this purpose. You can run the command
testfor @a[r=4]
from the commandblock in a loop using a clock, then check the output using a comparator. If you want a other center then the command block, there are 2 ways that you can do:testfor @a[r=4,x=X,y=Y,z=Z]
execute @e[type=ArmorStand] ~ ~ ~ testfor @a[r=4]
, you can give it a name and address it that way if you use this trick in multiple locations.