Here's what the Piazza Survival map looks like:
Here's how I got to wave 39:
During regular rounds stick claymores on the back stairs going up to the wine cellar, and sit at position 1. After about wave 15 or so you should have enough money to put a sentry gun and/or a sentry launcher at the far entry to the upper level of the wine cellar at the O.
Before the helo waves you have to bring your sentries back from the doorway or else they'll go offline in a heartbeat. When you have 1 helicopter left you should leave it in the air and go refill your ammo, get a riot squad (don't call it in until after you kill the helicopter), and get an air strike or predator or just go back to the wine cellar. Re-claymore the back way and
For the juggernaut waves stay at position 1 until the juggernauts get to you, then run down the back of the cellar and to position 2, where you can flash and kill some more guys. Then run up to position 3 which gives you another funnel point to flash/C4 or just get a couple rounds off on the enemies. Then run to position 4 and do the same. There are plenty of places like these where you can keep some cover while shooting the enemies, so keep doing that till they get to close, then move again.
Bad guys generally don't try to get into the wine cellar's bottom entrance (marked by the x), but congregate at the circled area.
So if you want to, before a round get some C4 and put a bunch in that vicinity, then wait for the enemies to bunch up there and detonate, and you will likely get some rampage/killstreak/quad kill money for it.
I haven't found a very good place for a turret, but these general rules should again be good for getting into the mid 20's in the waves.
Also, Piazza and Liberation both are Tier 2 maps.
Juggernaut/helicopter rounds start at wave 6 and are every 3 waves after that, alternating between juggernauts and helicopters.
TheMilitantPo3t's video strategy.