Did you actually read the info provided in the link you posted??

Address
Jassim Bin Mohammed St
Directions
Near the souk in the city's Al Bida district.

Did you tried to do a google search for that??

http://www.google.com.qa/search?hl=en&source=hp&q=doha+fort&meta=&aq=f&oq=

did you try to do a google map search for that??

http://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=doha+fort&sl...

Still need more details?? please contac QTA

http://www.qatartourism.gov.qa/contact

Regards!!