I'll start by saying that I took this entire project, from the concept to the building process, from the Terrain Wench's YouTube series, call "Squig Pit Unscripted Part 1-2." It can be found here:
I would suggest watching it, it is great. Also follow her channel. I liked the idea of a gated monster pit or corral. I figured it would go great with a Hordes army display. Here's my current progress.