I'm pretty sure its an British Army Enfield (short) rifle-musket. Designation is the P-1856, but officially known Pattern 1856 Army Short Rifle.
I did some reading, your rifle says TOWER just under 1857, which means it wasn't made by the Royal Small Arms Factory in Enfield but by a civilian contractor.
I found this, this guy has a musket that is very similar to yours.
http://www.svartkrutt.net/engp56.php
There is quiet abit of rust on the metal, I'd get it cleaned perhaps even professionally to avoid damaging it. Its 150 years old so be careful.
If you don't mind me asking, Where did you buy it? and for how much?