There are strategies using only 4th tier towers to survive DDTs, and some especially experienced players can squeak by with 3rd tier towers. And you want to put some thought into the towers you pick. For example, Cripple MOAB Sniper will easily handle round 90, but it can't handle the rush on round 95 by itself. And Guardian of the Forest Druid on a short map can't take out the round 90 DDTs by itself but can help out dramatically on round 95 by providing significant damage to all of the DDTS in the rush.