Territory Has To Connect

Every area inside the borders of an Alliance building counts as Alliance territory, and an Alliance Sentry Tower can push that border outward, but only into a tile touching territory you already hold. The tower itself also has to trace a path back to the Alliance Center, either directly or through a chain of other towers. Break that chain anywhere and the tower on the far side of the break goes invalid.

What Actually Stops

Two things ride on that connection staying intact. Alliance Resource Plots sitting inside valid territory produce Alliance resources automatically, the kind spent on Alliance buildings and Alliance tech. Members standing inside that territory get the growth buffs tied to it. Let the tower controlling that patch go invalid and both stop at once, the plots quit producing and the buffs quit applying, with no separate warning for each.

Castle Needs A Seat 🎪

None of this helps your own Castle unless it is actually parked inside valid Alliance territory. The buffs that speed up your growth only apply while your Castle sits on a tile the alliance actually controls, so a Castle sitting just outside the border gets nothing even if the alliance next door looks huge on the map.

Have you checked lately whether the tower covering your Castle's tile is still connected all the way back to the Center? 🗼