There are small windows on the way up to the Old Town Bridge Tower. There's a very good view from the windows over the Charles Bridge to the Prague Castle.
Photo tips: The windows are very narrow and deep, so it isn't easy to operate a camera, not mentioning a tripod. It's possible though and the result can be pretty good.
Sunset or dusk is the best time here, since the tower doesn't open until 10 am.
You'll probably need lens around 35 mm. Tripods can be very useful for late evening photography and are necessary for long exposures that can be used to blur the people on the bridge (and there's usually a lot of people even late in evening).
Note that you could get away without paying the entrance fee to the Tower, since these windows are before the cash desk, which is higher in the Tower. That said, you really should pay, it's quite cheap (100 CZK) and the way it's managed deserves to be rewarded. Also the paid sections are very worth the visit, be it the indoor parts or the top outdoor gallery.