Some noob questions

I randomly picked up the AoE2 HD from steam last night with my friend, and we duked it out, and it was great fun. Last time I played AoE was a very long time ago, I almost forgot everything about the game, and I was just a kid back then anyways. Now, I have a lot more gaming experience, and RTS experience in particular. I'm master in Starcraft 2, and have some concept of macro and stuff.

Now, my questions after just couple games:

1) I picked Saracines because I remember their signature camel cavalry being good. Turned out to be pretty good. What other strong unit comps do Saracines have? I pretty much rushed castle and mameluks, and that was enough to win.

2) Why do people only build one town hall? In other RTS, it's usually profitable to have several worker-producing structures to get ahead in economy.

3) What's the best way to defend against early agression? I tried towers, but they suck.

4) Does castle damage scale with number of units inside? Same for town hall.

5) Why are mosques and monks so brutally expensive? Are they effective high tier units?

6) How exactly do markets work? If it's 1 vs 1, is there even a point in making one? \

7) What is a good base layout?

8) What is optimal worker/army ratio? How many workers is optimal to have on the four resources? Is port with fishing boats efficient, or should I just spam farms?

9) Do all upgrades apply retroactively? For example, if I have an old farm, and make farming upgrades, does it work, or I have to destroy and rebuild it?

10) When I go scouting the enemy base, what should I look for? Worker count, tech, # of bases/expansions? What are the tells that I'm about to get my ass wooped?