Water drains a lot slower than you want it to.
There are three things to pay attention to - your "goblin catchment" area, getting the water in, and getting the water out. There are three design challenges - trapping a large number of invaders, flooding the area quickly, and draining it quickly.
For example, my current fort has this setup. My water reservoir is a 4 z level pit that is about 15x15 on the bottom level, with ramps at the edges & expanding outward as it goes up. The bottom is next to my Drowning Chamber, and lined with floodgates & fortifications. Then, the entry hallway/drowning chamber is floored with retracting bridges (five 3x10, which I'm currently expanding). The floor below the bridges is channeled out, to a "collection area", which is equal in size to the drowning chamber area, with many channels covered by grates. Then below that, I have a draining area which floods a large chamber, to dissipate water & allow underground tree farming.
The big thing to understand is how quickly water will fill or drain from an area - a few drain spots will take a long time to do it, which can be very bad if it's your only entrance. If you get a large head of water, the pressure will cause it to flood faster, and a large drain area will drain it faster.
Off to work now, but I might try to post some pics later today, if you want.