Gem Tower Defence is a castle defense Flash game, very similar to the Warcraft version of TD. Unlike Desktop-Tower-Defense, it not only includes an upgrade system but also integrates the concept of weapon element synthesis. After each round of attacks, you can place 5 gem blocks, at which point 8 different types and levels of gems will randomly appear, and you can only choose one to keep, the others will turn into regular stones. Although these regular stones can be removed for free, it's best to use these stones to form defensive works.