The Sentry Post is the cognate of the Outpost, but has an altogether different role: in addition to raising low-level foot mercenaries like the Outpost, it is also used to conduct research into Taxation. Like the Observation Post and Lookout from Rise of Nations, they also serve as an early warning system, capable of spotting enemy units even if they are concealed.
The Lydians can build these structures faster than any other faction, allowing them to access mercenaries far quicker than would otherwise be the case.
Although caravans passing along trade routes can collect wealth, tax however is the main way by which you can enrich your coffers with wealth.
Tax ensures that you can collect money from any active economic activity which goes on in your realm, as well from the percentage of territory you control on the map.