The last thing you do is drop rat. It’s on fire, so you don’t need it any more. But remember where you leave it because that marks the exit. Don’t leave yet. You have seven more room to investigate to find the lost key. You can’t leave without the key.
Right before you drop the rat make sure you have the bag of sugar. The ants love sugar and you can use the sugar to bribe the ants. They can do crazy things like tunnel through the mortar between the bricks to make doorways that weren’t there before, and you can send out a pair to scout each room before you enter. When the work in pairs they are less susceptible to getting off task. Ants are prone to wander when left on their own.
Most monsters will ignore the ants, except for the giant fire-breathing anteater, which is how the rat caught on fire. Did I mention the fire extinguisher? If you brought it, put out the rat. If you don’t have it, go back and get it. We’ll need it when we meet the anteater again.