Solid
A servant secures a stool in the dirt so that it is a solid sitting device. If you just throw a stool on the ground it will flop over easily. But if you have a servant to dig it into the dirt a bit, it becomes nice and solid and safe for sitting!
Picture two stools in your mind. One is solid, you want to sit on it. The other looks like the wind will knock it over. Don't sit on that!
けん
かた
None
You need this stool to be extra solid because you're using it to test your new catapult (かた). If it isn't solid, when you launch it and yourself in the catapult, the stool might fall over, killing you both. And you don't want that, now do you?
You better hope that catapult is solid too. If it isn't the whole thing will come apart when you try to launch yourself into the air.