exactly what it sounds like, some levels unlock an endless wave of orcs while you try to last as long as you can. they're campaign levels but the orc units are mixed up and it's a good way to earn extra skulls while practicing mid-battle spending habits.
John, in the traps menu if you click on the regular Wall Arrows it takes you to the upgrade page, the third down allows you to place them on ceilings :) every trap/weapon has three upgrades available be it damage, stun, location or whatever. Provided you've got the skulls of course.