Team-Fortress 2 style sentry gun add-on.
Add-ons are expansions to Blockland. Add-ons can be downloaded from the Blockland forums, or from the now Discontinued Return to Blockland, which is itself an add-on. There are a total of 1,965 add-ons to date on RTB, nowadays addons can now be downloaded from Blockland Glass, a service similar to RTB, but made by Jincux.