I've had the game for about a week, and I discovered a little trick that allows me to consistently beat the computer by 1850AD (with the spaceship, longer if I fight).
Usually, newbies get overrun within a few hundred turns when the computer sends out ironclads to soften up your cities, and then sends in invaders. The trick comes in a few parts:
First, turn off global warming and then make 0 coastal cities. (If global warming is on, suddenly your "safe" cities will become vulnerable to naval bombardment.) It also helps to set phasemode to "player". Spacing doesn't matter, but I usually put the cities 3-4 squares apart. Putting the first 2-3 cities within 2 spaces of the capital speeds up the game somewhat, but it doesn't matter. Usually, I prefer to have bigger cities later in the game than shaving a few rounds off of getting Republic.
Second, when the AI sends a loaded (look for the "+" sign) transport, immediately switch all production to barracks and two of your strongest defenders (phalanx or musketeer) that you have. Even on "hard", the computer will completely pass you by, usually giving you a cease fire treaty additionally. If they don't unload, you can stop construction. If they try again, bluff again. Just make sure you have all three (barracks, 2X defender) in your construction queue. Also, if you get a legion from a hut early, it's good to charge any trieme as it passes in case they try to drop off a settler.
If they do unload, usually it will be a single cannon (if you have musketeers,) or musketeers (if you have phalanxes.) At the cities closest to the cannon, immediately switch to city walls, buying them if necessary. At the next group of cities, switch to barracks and defenders, buying them if necessary, and sending them immediately (within 1 turn on roads, hopefully) to the first city. Hopefully, you built some combination of these already so it will be easier. Usually, it will take ~3 turns for their unit to get to your city. If there is 1 veteran defender and city walls, they will usually find a forest or hill and sit there. Then, you can leisurely build a catapult to destroy it.
Around 1000 AD, the computer will immediately attack anything along your border with ships. After a certain point, you can't even send workers to irrigate, so take care of that early.
Around 1200-1500 AD, the computer may start using paratroopers, so you will have to defend every city with 2 of your best defenders and city walls. Without naval bombardment, the computer won't even try to attack your cities.
I tested this out with a 9 city island on novice and a 5 city island in hard. Apparently, the AI has two weaknesses you can take advantage of: the ability to see into your cities allows you to fool the computer into thinking you are going to buy a bunch of defenses, and its reliance on naval bombardment means you can buy about 200 turns of relative peace.
My typical research order:
Pottery (usually I get this for free from a hut. Make sure you set your research as soon as you start a new game!)
Republic (make sure you get Writing before Code of Laws)
Ceremonial burial (get it just before or after Republic)
Gunpowder (make sure you get the Masonry requirement first)
After that it doesn't matter. University and Refrigeration are nice, but I would be very careful about getting Mobile Warfare because your barracks will disappear and you do not want to get stuck with non-veteran troops. When I build them by accident, I usually immediately disband them.
Since you can't make boats, don't bother with the sailing techs. Rapture is unnecessary as well, you mainly just need to remove the minuses of despotism. Usually, you'll be switching to Democracy within 20-50 turns anyway (2X as much research points, on average.)
Note that getting a city from a hut in the wrong place will screw you up. I usually save/load before opening a hut in case I get a coastal city.
Also note that being landlocked is not irreversible. You can transform land into ocean and create a coastal city, or build a city on an isthmus.
Side note: I also found a trick to getting tech fast with despotism. Currently, the guides state to build your early cities within 2 squares of each other to lessen the effect of corruption. In general, what this means is you will get 2 instead of 1 science per city, or about twice as much. However, if you have 5 cities jammed into an X pattern, you will get exactly 10-12 tech. On the other hand, if you make one of your citizens a scientist, he makes 3 science tech. If you have a library, this becomes 6 tech by himself. If your city is producing 1 science, you get two science because of the library, and thus you now have 8 tech, 10 tech at your capital city. Therefore, if you have 3 cities with 3 population each with libraries, and each third citizen is a scientist (and 0 extra food and no growth,) you will be making 26 science per turn. I usually get Republic by 1500-1200 BC, my record with this technique is 1850 BC. Just remember that each scientist is a flat rate, so it doesn't matter when or where you do it. However, it's hard to get more than one scientist unless you have grassland or wheat, but this sacrifices production. Also, 3 cities seems to be the right balance between fast research and having size 13 cities in the late game. It also speeds things up if you disband the explorer and any legions you get from huts to help build/buy granaries and libraries (of course, once exploration is done.) Even with only 2 citizens working, you can still build barracks, workers, etc.