they should have a monster that has a portal gun!!! or like a catapault option wheree you shoot a portal at a place, and then all the portal monsters teleport there
Your turrets are spread to thin, you can't help it at your level but focus on overlapping mortar and gun turret ranges. Really a good base just needs to have overlapping defenses. Personally I overlap my towers in pairs and then throw in armor, heavy gunners and rocket launchers for good measure.
Mudkipz the water pokemon|:worker: :pokey: RUN WORKER, RUN! pokey's going to pwn you