Open the Encyclopedia in-game

Only way to "split" an army seems to be having two generals. When you order a general to march to another general, you get a possibility to change troops from on army to another.

Compared to other TW games this seems like a huge drawback to be, because you dont seem to be able to recruit troops to undefended city if there is no general present nor can you leave troops to defend a city when your army marches on. City garrisons are usually not enough to defend a city and you need to carefully place your armies.

It does have a certain point, because roman armies were strictly hierarchical and there had to be a general leading it. It leads some inconsistencies tho, like in case your general is killed, you will be able to recruit a new one to that spot immediately.

The difference is in the ships and number of troops.

Naval troops are fewer than their land equivalent, making them weaker as a result. However, naval ships are far more powerful than transport ships and also come in several variations.

Some of the ships can destroy an enemy transport by ramming it only once even frontally, others can be particularly fast, allowing you to flee from or intercept any transport ship. Also, siege ships built in the harbour actually have siege weapons. A siege transport on the other hand only transports the troops that man the siege weapons, making land siege units pretty useless in naval battles; similarly, a cavalry transport only contains the riders, not the mounts; such ships cannot be built however and only exist as transports.

TL;DR

Transports:

• Have more troops, and are therefore more suited for boarding.
• Have weak ships, and should therefore avoid being rammed.
• Do not allow the use of siege weapons or mounts, which need to be protected at all costs.

Military ships:

• Have stronger and faster ships, making them ideal for ramming or hit-and-run tactics.
• Have special skills, which transports don't.
• Can use siege weapons, giving them a significant advantage over very long ranges.