Attack and Defend (AVD), or Invade and Defend (IVD) is a game mode in the Team Fortress series. The objective of a AvD map to capture control points in order by placing a flag on them. Usually, the Blue team attacks and Red defends. If all points are captured, then the attackers will win and either the map will end or the game will switch the teams and start again, depending on the map. All AvD maps have a time limit. If the limit runs out, then the defenders win.
AvD maps are significantly different compared to maps in other gametypes. Instead of there being defined "bases" for each team, the entire map is a battleground. Some maps, such as Dustbowl, split the map into several pieces to make the map more manageable for both teams, while others, such as Avanti, do not.
Many AvD maps have small chokepoints that can be spammed with grenades, both thrown and from the Grenade Launcher. these points can be broken thoguh, but it requires a concentrated push from the entire attacking team. The defenders can also run out of grenades, which allows the attackers to push while the defners are restocking. Attacking Demomen can also be useful, as they can bounce their grenades past the chokepoint and hit the defenders.
Unlike other modes, almost all classes are useful on offense and defense.
Offense should consist of a mixture between slow and fast classes. the slow classes should be used to clear the way for the faster classes that are carrying the flag. Engineers are surprisingly useful, as they can use their Teleporters to qickly get allies to the battlefield, their Sentires can be used to hold a perimeter and Dispensers are useful for classes that eat up ammo, such as the Heavy.
Spies can be very useful if used efficiently. By sneaking past the enemy's front line with their disguises, they can inflict major damage on the enemy by backstabbing key defenders, such as Demomen, and by blowing up Engineer buildables with Frag Grenades.
Defense should mostly consist of strong classes, such as the Heavy, Soldier and Demoman. Engineers should build a Sentry or two near the capture point just in case a speedy attacker slips by most of the defenders. However, a Sentry or two should be placed at the front lines to give the front line more firepower. A dispenser can be very helpful for ally Engineers and allies that need to stock up on ammo and armor.
Medics can be useful for keeping the front line going. A quick healing to the front line might make the difference between an enemy's push working and failing.
As with the attackers, Spies can seriously hurt the attackers. They should target powerful classes and Engineer buildables (especially the Teleporter) so that the flag carrier can quickly be destroyed by the other defenders.