Just a heads up, I changed the Sentry Control objective to an Alien Artifact temporarily. The sentries spawned had a bug where if you tried to attack them you'd be brought to an error page, unless you first refreshed your page before clicking Attack. With sentries moving in and out of squares it became really difficult to even attack them.

Sorry for the abrupt change, but I think it's the best choice to preserve everyone's sanity until the bug gets fixed.