So last session the players spent the time to establish a proper trail along the path known informally as Gravemarker Road – a convenient line of cairns, marking where several deceased PCs are buried, that leads from the Woodcutter's Camp to Drop-Off Tower. They also spent gold securing the first two floors of the Tower to use as a forward base of operations.
From a DM's perspective, this is an awesome development.
At its basic level, it shows the players (not just the characters) have investment in the game and were willing to spend an adventure session building infrastructure. For a DM this is a most desirable piece of feedback from the players – they are interested enough to start making the place comfortable and convenient. Second, it allows me to start having the locals actually take notice of the adventurers. This opens up the door to several role-playing opportunities that would have been more forced while the PCs were just foot-loose vagabonds.
The earliest versions of the D&D rules assumed that as the PCs gained levels, they grew in importance and influence. They had mechanical methods of establishing this – at 10th level characters gained access to a stronghold of some sort and followers. There was no direct linkage to what the PCs had been doing, just that they reached a certain level and "bing!" they got the accoutrements of power. This disappeared in later editions (I can't remember if it was 2nd or 3rd edition where this disappeared, but I'm inclined to believe it was 3.0 that did away with it) and I never really missed it as it seemed too metagame for my taste.
With the PCs in the Southern Reaches actually making improvements to the area and establishing a place that is definitely theirs, I now have an in-game mechanism to support the meta-game mechanism. Agents of the Duke will be making contact with the PCs in the not so distant future, asking about their intentions. Plus, I've had some ideas about the PCs being agents of change for the area, leading other groups to start establishing themselves outside of the Keep through example. Some of these groups may even be benign. Heh.