Question Regarding Adwords Display Network - Targeting

When you use "Targeting" on the Adwords Display Network, can you do "Placements" only...or do you also have to include "Keywords."

I was thinking that as long as my ads appear anywhere on the Placements I determined I'd be content, and that adding Keywords would only limit distribution. However, since removing the Keywords I've not seen anymore impressions/clicks. This may be a delay in their reporting, but I wanted to see if anyone knew if specifying Keywords was necessary in the Targeting section?

It's difficult to get distribution when targeting only a small amount of placements. If you have a large list it's more likely.
But I find it's best to keep all keywords in place, and instead block any placements that aren't performing well for my campaign.
This should keep the volume high.