do i need heroes to defend a town? or can i use an army instead

You can defend town even without a hero, although it is better with one (attack/defense skill buffs and spellcasting).

thanks because i used heroes to defend but thats the problem i have to use a week building refill in town to make some defense since a town siege got most casulties since the town has archers on towers as defence and mostly if i take a town the computer has already used the soldiers from there to replence his army.

You can also leave garrisons* at mines. If an enemy hero with a tiny army is roaming around you can leave a small garrison at various mines and force the opponent to send a REAL army. Then the enemy is taking an actual risk of losing something if you catch his hero with a superior force.

*how? Put a hero on top of the mine, hit the space bar. This "hit the space bar" also works for skipping back and forth on top of a teleport gate, dungeon entrance or city entrance without wasting movement points by moving back and forth.

ref defending towns. leaving a garrison works - HOWEVER, if you're in a scenario where the enemy has high level heroes, then his armies will easily crush garrisons of equal size. To have a garrison defend a town successfully against a high level hero the garrison would have to be so large that it's not worth it. In those situations it's easier to take back the town and kill the enemy hero with a bigger army led by a decent hero.

Wow, I didn't know this. Thanks for the good tip.

yeah thanks a lot m8

Neutral armies joining is a bit more complicated than that actually.
First, any unit not marked as "always joins" will always fight you if it's not much weaker than you.
Second, diplomacy both adds a big chance to neutral units joining for free (about 20% from my limited tests) and creates a chance for any neutral unit to be recruited for money.
That makes it one of the bet skills in the game imho, especially in maps where you can have a large starting army but limited resources to hire more, or where you have lots of gold but not a lot of creatures to hire.
Third, the chance for neutral units to join is boosted if you have the exact same unit type in your army already (upgrades don't work).

I used to think diplomacy sucked, but I was wrong. It's great. Lots of reasons.

Eventually you'll capture castles of all different types. With diplomacy, you can put together enough troops to defend different types without being constantly bankrupt or having to build so many creature buildings first. That can be a real load off your mind, and not having to run back to defend castles all the time, or park useful heroes in them, saves tons of time and lets you get around the map and develop your heroes a lot more.

It's also great if you like to save up lots of stacks of enemy creatures around to level up secondary heroes on later. When you get various levels of diplomacy, save first, then attack. If they join and the stacks are still small, don't waste a valuable future asset. You can save them for later, when they will join with 200 or 400 instead of 30. Reload and leave them alone until there are tons of them, if you can. They'll also give more experience when the stacks are bigger, so finding out which stacks you want to fight is very useful. If, however, you're at expert diplomacy and approach a big stack with a strong army and they still don't join, load the secondaries you most want to develop up with plenty of tough troops and have at them, leveling them multiple levels at once.

Totally sweet if you're a necro, too. Accept a bunch of low-level troops and turn them all into skeletons at the skeleton converter -- boom, tens of thousands of bucks saved at a time, instant armies for yourself or your secondaries. If you have the Cloak of the Undead King that lets you convert a percentage of all slain on the battlefield into liches instead of skeletons, you can build massive armies of liches (or power liches) really quickly by fighting anyone who won't join. Running around with two or three stacks of liches at a time is pretty aswesome too. Massive armies of skeletons may not fight until the battle's half over or already decided anyway, but massive armies of liches are godly. It also lets you leave your castle-generated weekly liches alone, overcoming a big weakness of necros -- only one ranged troop, and you are hard-pressed not to take it along with your hero, thus leaving your castle much harder to defend.

Anyway, diplomacy is now a priority when I play. Too good to pass up, and it sometimes saves me hundreds of thousands in a game.

If you ever learn the spell Armageddon I recommend that you select a brave and valiant hero that knows powerful magic and give him only Black Dragons to fight with. Then you can unleash katabatic winds of ravening doom and let your dragons devour what is left.