A very clean and relatively bitter beer, balanced by some malt character. Generally darker, sometimes more caramelly, and usually sweeter and less bitter than Düsseldorf Altbier.
Most Altbiers produced outside of Düsseldorf are of the Northern German style. Most are simply moderately bitter brown lagers. Ironically "alt" refers to the old style of brewing (i.e., making ales), which makes the term "Altbier" somewhat inaccurate and inappropriate. Those that are made as ales are fermented at cool ale temperatures and lagered at cold temperatures (as with Düsseldorf Alt).
DAB Traditional, Hannen Alt, Schwelmer Alt, Grolsch Amber, Alaskan Amber, Long Trail Ale, Otter Creek Copper Ale, Schmaltz' Alt
You can view the complete style details on the BJCP website. The 2008 style guide is obsolete but there is a similar 2015 style that you can view below...