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I finished this game some time ago, I'll try to answer some of your questions 
1) I don't recall seeing this hydra thing in the game
2) The tripods are present in the game, you also have to fight 2 of them simultaneously at one point. To destroy them you need a good bazooka and to play a lot of hide & seek 
3) The tower is the "bad" aliens HQ, you'll enter it and go to the top. You'll have a close tripod fight inside !
4) The old man is supposed to be the highest ranking human in the alien organisation, completely sold out to them, he is on every monitor all over the city to spread propaganda (directly toward you at the end)
HL2 is fully scripted, meaning that's it's totally linear. Do not expect any freedom.
And there isn't any end level bosses, hard passages are well brought, like when you get surrounded by ground and aerial forces in an isolated farm with your fellow resistants. There are helicopters, explosions everywhere, a real battlefield ... thoses passages are nice |
