Without any ability to experiment, I want to know some things they are not addressing -
1. Why would they use a tree if they had siege engines?
2. How did the invaders cart plague ridden bodies about without contracting the plague themselves? Hazmat suits? And what good would it do them to take over a city that now has plague? (assuming you don't just want the other guys dead, and you want at least the ability to loot.
3. Castle walls don't usually have available trees growing right outside, do they? And the trees they are testing (Douglas Fir, Alaskan Cedar....) don't seem like likely candidates to run into in Medieval Europe to me
On the plus side - the castle they are using is a bouncy castle. I approve.