Fallowfield (pictured) joins the club having previously played for former National League North rivals Harrogate Town and Hull City.
The 20-year-old had his contract at the KC Stadium terminated by mutual consent in March 2015.
He had earlier moved on loan to Harrogate for a three-month spell in December 2014.
During last term he was sent out on loan to Evo-Stik Northern Premier League Premier Division side Matlock Town.
Armstrong joined Ferriby in the summer of 2015 from NPL First Division North side Ossett Town but spent most of the season out on loan with the likes of NPL Premier Division Frickley Athletic.