  Building structures by low capability robots is a very recent research
development. A robot (or a mobile agent) is designed as a deterministic finite
automaton. The objective is to make a structure from a given distribution of
materials (\textit{bricks}) in an infinite grid $Z\times Z$. The grid cells may
contain a brick (\textit{full cells}) or it may be empty (\textit{empty
cells}). The \textit{field}, a sub-graph induced by the full cells, is
initially connected. At a given point in time, a robot can carry at most one
brick. The robot can move in four directions (north, east, south, and west) and
starts from a \textit{full cell}. The \textit{Manhattan distance} between the
farthest full cells is the \textit{span} of the field.
  We consider the construction of a \textit{fort}, a structure with the minimum
span and maximum covered area. On a square grid, a fort is a hollow rectangle
with bricks on the perimeter. We show that the construction of such a fort can
be done in $O(z^2)$ time -- with a matching lower bound $\Omega(z^2)$ -- where
$z$ is the number of bricks present in the environment.
