[akots]

If you point to the port building, the cursor should change into blockade.

With vanilla campaign, it is quite hard going in the beginning. Just take it easy and slow but accurately. Medium difficulty becomes very cheesy quite soon since Roman infantry is way too god both in combat characteristics and in morale when positioned properly and timely moved on battlefield. Just don't get yourself surrounded and you cannot lose even against overwhelming odds on a higher ground with a good general.

With gallic hordes and with most other barbarian hordes, the easiest way is to sandwich them between infantry holding the line (hastati and velites are OK but principes are superb) and hammer them in the back with a couple units of cavalry or hastati. They rout almost instantly and it becomes pure slaughter. Having a good general in command is a must though. In case of several large armies marching in, it is possible to give them a well fortified city with a reasonable garrison to besiege and then attack them from within the city. The AI can be easily lured to the walls and will get slaughtered by the defenses and archers. Barbarians seldom build seige equipment early in the game. Even if your sally cannot win the battle, it is going to be a draw if they don't take the city but they will lose a bunch of troops. Thus reduced army can then be defeated easily in the field with a modest Roman force.

Economy needs trade and the most income comes from port cities. Since naval combat is completely broken in vanilla, it may be not worth to build a lot of ships because they get slaughtered by AI armadas. So, it might be better to pursue the lay of the land and just capture these ports using ships only for transport. At some point it gets tricky but eventually, you can generate so much money that happiness becomes easy to maintain even in remote provinces. Also, income generating buildings and roads are of course top priority.

I don't know what is the best mod now, but I played a lot of Total Realism starting from version 5. It is substantially more complex and fixes a lot of exploits apart from being able to play all civilizations and different larger map(s). IIRC, I played Seleucids, Egypt, and Greek Cities to the end which was a great fun. It is also vey enjoyable visually with high quality graphics and huge variety of units and well thought through tactics.

Also not sure about latests patches. IIRC, community patches from player1fanatic were very comprehensive and fixed most of bugs even in the latest patches.