My city is out of the protection week. I do not have a castle built yet. I was wondering how I should build my towers? What mix makes sense?
It depends on time, your aggregate production allocation, your financial engagement and few other things.
This game favours those willing to spend some real money, change the game or be ready to use many extra hours for monotonous activity such as pressing certain keys over cities. Even if you were able to automate the latter action with user-space tools (alert against ToS), notice that the ready tools with ministers are much better. I am not saying that it is not possible with OCR engines and certain window automation tools, but user-space hacks are not built-in and EA is acting violently against such maneuvers, as you may have noticed with LoU Tweak/BoS not-working-parts from userscripts.org and no public API.
You can ask for a friend for a holiday substitution, but notice that it is only for a limited time and with limited functionality (no trading, no attacking). I have talked with many past LoUs -- and guess what -- they have been using ministers during the whole gaming. When you have about 40-50 cities, it becomes increasingly more difficult to advance without real financial engagement. It is still easy when you are not under attacks but during attakcs everything becomes even harder -- without ministers welcome unhealthy 24/7 gaming. Since military is pretty much an exponential cost-curve (or if you have war-minister, you can get income with automatic raiding), you can just imagine how the amount of different factors greatly increases because you need to create more and more resource cities to please standing armies. Playing as poor and playing as rich are very different activities. It will affect greatly your choices over possible defence strategies.
Land-locked and harboured cities are very different beasts. If moongates are open, the value of land-locked cities may deteriorate after the invasion of strong enemy units from other continents. If moongates are not open, the value of water-based cities is lower in terms of attacking because you will face 2 types of attacks: water-based and land-based. Luckily, water-based cities provide one of the strongest defensive unit in the game: sloop.
Capacity and Real money
People willing to spend more money can take more risk. They can plan poor cities with low capacities because warehouse cities can suck the resources or a minister will purify things automatically for you. If you have no minister, you should aim to create cities with about 2-4M capacities and implant every city with a farm. The farm is important because it kills one factor: you do not need to worry about sending stuff to your cities the hardness of which exaggerates while under attacks.
It is very hard to say which type of defence to create for a pause. It depends on a number of issues. A key issue is financial engagement which undermines the expertise of this game, those willing to pay a lot have a huge advantage. For a player unwilling to pay a lot, I recommend to balance things (create a balanced mix of traps to resource cities for example), build more capacity and implant a farm to every city. Even if EA tries to give an impression that you can get most things without money, you must understand that the game will never end, it just expand with new continents (and new world) and old player getting killed after inactivity.