Spider

The spider is an agile and speedy mob. It has a plethora of features that adds to this. It is by far the speediest mob, and has a pounce that allows it to cover several blocks at once

The first of these is the ability to climb walls. They can climb cacti, glass, stone etc. But there is one way to stop it, which is add a ledge to your walls. A simple lip will stop spiders from scaling your walls, or alternatively you can add a ladder to the top layer of your wall. The wall scaling ability is made even more efficient because now spiders can see through walls. With this, the spider can see you inside of your base, which can be quite a security threat if you have walls rather than a roof/house. When attempting to kill one, a bow will help as they are a huge target. If you kill one, it will drop string. But note you need not always engage them, as even if they don't die in the sunlight they become peaceful during the entirety of the day.