Comment 12 for bug 732806

I am not sure if the scripting is currently up for this idea, but here is a suggestion; untill the player has built a donjon and resolved the event, only allow the player to build sentries close to the headquarters. I think what we want to achieve is that the player builds a donjon in order to progress close to the tracks. So an alternative approach would be to mark the area closest to the HQ (could simply be based on a radius/x fields from the HQ). Inside this area, sentries are allowed for inital expansion, though outside this area it would not be allowed untill the event is resolved. In other words, sentries can be used to expand for a while, but then the player is forced to switch to a donjon.

In the case where a player doesn't expand, the sentry is available as a buildable building, but when the time-based event is triggered, this choice is suddenly removed. I think it might make sense to still allow players to expand the intial bit with sentries, before a donjon. This would also make it somewhat closer to what happens if you expand from the start and find the tracks on your own.

Might need some tweaking, but could be something to consider. In addition to whether this is currently possible, we might have some issues with defining the area; both to not prevent expansion further east which should not interfere and restrict it so that the player cannot simply build around the tracks and skip the event.