Jamestown was laid out with considerable precision: a rectangular circuit of walls, two main gateways, a postern gate, and four corner-bastions enclosing a single north-south street, all to a specification measuring roughly 370 metres by 150 metres. By 1622 or 1623, a government report was already noting that six or seven English houses of lime and stone had been built and more were under construction. The place was not a casual settlement but a deliberate instrument of colonial control over one of the Shannon's important crossing points.

The town was incorporated by charter in 1622 and its walls built under the direction of Sir Charles Coote. A medieval church, recorded as Cill Srianain or Cill Trenain, may already have stood at the site as early as 1310, but the walled plantation town was an entirely new creation. Coote also erected a wooden bridge to the south, still standing in 1683, before it was replaced by a stone bridge by 1730. The town's position at a river ford made it a target repeatedly. Rebels besieged it in 1641 and 1642 without success, but Confederate Catholic forces took it in 1648; a meeting of Catholic prelates was held there in 1650. In 1689 it changed hands twice in the same year, captured for King William by Colonel Lloyd and then retaken by Patrick Sarsfield, for whom the Shannon crossing made it worth the effort. A mid-seventeenth-century barony map from the Down Survey depicts the walled town accurately, but also shows a second bridge northward to a projected county town for Roscommon at Charlestown across the river, a scheme that was never realised.

What survives today is fragmentary but legible. The North Gate still stands, along with two sections of town wall, the church, and the star-shaped fort on a drumlin to the west, a drumlin being one of those smooth, oval hills shaped by glacial deposits, common across this part of Ireland. Excavations along the main street in 2007 turned up cobbled surfaces laid over an organic layer containing pottery, glass, oyster shells, brick, charcoal, and bone, all dating to the seventeenth and eighteenth centuries, the physical residue of the town's brief but turbulent working life. The site of a Franciscan house, probably established after 1645, also falls within the old walls, though its exact location remains uncertain.