Either way, I would appreciate it if you could point out flaws in this simple design, give suggestions on better designs, etc.
I'm afraid I can't point out flaws in that simple design. You see, the flaw in that design is that it's not simple.
Another flaw is that your initial pressure plate - the one the enemy steps on - is going to have to be some distance back from the bridge, because bridges don't operate immediately. I'll show you the automatic enemy blocking mechanism my last fortress used at its gate. Here's the main gate (approximately):
#...#
#...#
#^^^#
#¢¢¢#
#╔═╗#
#╚═╝#
#...#
Each of the pressure plates (^) is directly connected to all of the hatches (¢) and to one more hatch elsewhere. Elsewhere, you see this:
#.#
#.#
#┼#
#^#
###
Directly above the pressure plate in this diagram is the hatch, and directly above that hatch is water fed from my reservoir. That is, there will always be 7/7 water above that hatch. The pressure plate in this diagram is connected directly to the bridge in the above diagram (and also to other defenses that should kill the enemies who stepped on the gate's pressure plates in the first place - otherwise they just disabled your gates completely.) The door in this diagram leads to a drain; it is the only reset needed.
Inside my fortress you see this:
╔═╗
║ò║
O^O
The lever does nothing - it's just to trick the dwarves into walking across the pressure plate. The pressure plate is connected directly to the door in the above diagram. (I do this so that I only have to manually designate pulling the lever once, instead of twice.) In short, the entire system can be reset by ordering a single lever pull.
So, when an enemy steps on one of the gate's pressure plates, the following sequence happens:
1) The gate hatches open instantly and the enemy is stopped. Meanwhile, the lasting-effect pressure plate gets 7/7 water dumped on it, and
almost immediately triggers the drawbridge to start closing.
2) The enemy steps off the plate, but by the time the plate deactivates and closes the four hatches, the bridge is up.
3) The other defenses kill the enemy.
4) I notice that some enemies have set off the defenses and allocate the pulling of a lever. The door opens, the water clears out, and the lasting-effect pressure plate turns off after 100 frames. The bridge closes another 100 frames after that, and the violent defenses turn off. Everything is back to normal.
With a bit more cleverness, you could add overrides and/or prevent the system from wasting water if you pull the all-clear when there's an enemy on one of the pressure plates.