This conception is an idea in progress. Please refrain from criticizing it too much, as what you think may not be true in the final version.
Web Caves is a special advanced track unlocled after beating Skull Shores. It looks like a blue rocky place with spider venom "rivers" in it. Some crystals can be seen on the rocks. There are spider webs wherever possible. It has 5 entrances and 5 exits. At random moments in a round, an Arachnedean Bloon may appear from under one of the webs and attack the nearest tower. It is only freely playaple after beating its Special Mission.