It simply won't show the option to build a Sentry. I filled the places around my HQ with basic buildings and waited for the trees or stones to fall so I could build a Sentry, but even now, that I have some space, I can't build it, it won't show on my building options. What do I do?

SirVer wrote: Most likely you have the objective to build a barrier (or donjon?). Sentrys are disabled for as long as you have not build a barrier.

Yeah, that's my quest, I just didn't knew that needing to build a donjon would not let me build sentries. Thanks for the answer!