I would aim for 'wealth' then at least 'expansion' from the seafaring side.
Wealth, like AD said, will let you start gathering the unique resource of the island (wine, marble, crystal, and sulfur) and expansion will let you build a colony (which you shouldn't yet) but more importantly will let you send trade ships long distance.
Long distance trading + surplus luxury resource = Trade with
TG members for needed resources (like marble and wine).
Once you've got the ability to do trades over the horizon you can barter with your surplus for the specific needs you have (like crystal for research, sulfur for your army, wine for population growth, and marble for building upgrades). This will dramatically increase your growth potential.
Depending on how hostile your environment its a toss up then if you want to go for foreign cultures to get embassies and a spot on the TGA waiting list, wine press for taverns so you can spend wine to increase your population growth, or professional armies to gain access to swordsmen and phalanxes.
Getting into TGA will be the BEST defense you have against pillages. Nothing says 'hands off' like an alliance tag for a prominent alliance. Also, the longer you delay the longer the waiting list gets to get in.
Wine press will cause your population to explode once you have a nice stock of wine on hand to keep your taverns running. Faster population growth means more citizens. More citizens means more scientists and workers. More scientists, workers, and citizens means faster research, more resources, and larger income. All that leads to faster growth overall and 'purchasing power' over trade markets to better acquire what you need.
Lastly is professional armies which gives you access to some of the 'staple' units of your military for a while. Until you get ballistics your swordsmen will be your primary attack unit and your phalanxes may never get phased out until you research steam robots for the defensive side (high defense + wall bonus + resistance bonus + workshop upgrades whenever they start working = the proverbial hard place against your rock walls). To be honest though, I consider a land defense the least important part of your defense because at this stage in your growth you really have nothing worth losing. You've already researched ballista ships as the first tier of seafaring and a single ballista does more to deter enemies than land forces generally do since no land forces can invade as long as you have ships protecting your harbor. Of course they can always blockade your harbor, but again if you're at that point then no number of phalanxes will defend your city because they're determined to pillage you and have done their homework.
