In the beginning of December 1642, Fortescue summoned the posse comitatus of the county to meet him at Modbury, in order to join Sir Ralph Hopton, who was then marching from Cornwall to besiege Plymouth.
About two thousand men answered the summons and assembled on 6 December, intending on the next day to join the main army, whose headquarters were at Plympton, only three miles distant.
During the night, Colonel Ruthven, commanding the Parliamentary forces at Plymouth, organised a sortie from that town of some five hundred dragoons, who, avoiding the village of Plympton, fell upon Fortescue's Trained bands at Modbury.
They then proceeded to Modbury Castle, a seat of the Champernowne family, fired the house, broke in and took prisoner Fortescue, his brother Peter, Sir Edward Seymour and his eldest son, M.P.
On 9 December 1643, Fortescue received a commission from Prince Maurice to repair "the Old Bull-worke near Salcombe, now utterly ruined and decayed," and to hold it for the king.
The inventories of provisions given in this account show that nothing necessary for the support of the garrison during a prolonged siege was neglected: more than thirty hogsheads of meat, ten hogsheads of punch, ten tuns of cider, two thousand "poor jacks," six thousand dried whiting, and six hundredweight of tobacco, are among the items of the provisions supplied, while such entries as "twenty pots with sweetmeats, and a good box of all sorts of especially good dry preserves," one butt of sack, and "two cases of bottles filled with rare and good strong waters," show that Fortescue did not forget to provide for the table of the officers' mess.