Only one thing i see right now is about the water on the 1st 2nd and 3rd pic, make em Func_water and change the fx_amount to be like 90 , so its clear

Oh an orph i agree, Jez i know most of us here have used, or still use dial-up and it really does make a large difference ^_^.

The scenery in the first shot looks pretty interesting, but elsewhere it comes across as rather blocky and simplistic. Perhaps you could have added additional landscape features above the current ceiling line but kept them inaccessible - rock spires, arches, etc... Of course this is HL1, and I've no idea how well the maps runs with what there is, so I'm possibly being unrealistic. In the second screenshot it looks like you have some sort of flame sprite on the bridge ends, but they aren't fixed axis sprites and so rotate with the player, looking awkward.